With the rapid advancements in technology and growing aerospace applications, there is a need for effective low-weight and thermally insulating materials. Aerogels are known for their ...ultra-lightweight and they are highly porous materials with nanopores in a range of 2 to 50 nm with very low thermal conductivity values. However, due to hygroscopic nature and brittleness, aerogels are not used commercially and in daily life. To enhance the mechanical and hydrophobic properties, reinforcement materials such as styrene, cyanoacrylates, epoxy along with hydroxyl, amines, vinyl groups are added to the surface. The addition of organic materials resulted in lower service temperatures which reduce its potential applications. Polyimides (PI) are commonly used in engine applications due to their suitable stability at high temperatures along with excellent mechanical properties. Previous research on polyimide aerogels reported high flexibility or even foldability. However, those works' strategy was mainly limited to altering the backbone chemistry of polyimide aerogels by changing either the monomer's compositions or the chemical crosslinker. This work aims to summarize, categorize, and highlight the recent techniques for improving and tailoring properties of polyimide aerogels followed by the recent advancements in their applications.
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•Exploration of PI aerogels as new class of super thermally insulation materials•A detailed discussion on PI aerogel processing and polymerization steps•Summary of emerging techniques for improving the performance of PI aerogels•Perspectives on the opportunities and applications of PI aerogels
A nanocomposite strategy for the combination of a polymerized silica precursor, such as polyvinyltrimethoxysilane (P-VTMS), together with electrospun thermoplastic polyurethane (TPU) nanofiber in ...the aerogel backbone is demonstrated to create effective stress transfer pathways in three-dimensional (3-D) aerogel composites with thermal insulation characteristics and special porous structure. Inspired by the bone architecture in the human body, with large amounts of hard segments and small amounts of soft segments, the 3-D interconnected TPU-embedded P-VTMS-based composite (P-VTMS/TPU) aerogel achieves synergistic strengthening in the nanofiber orientation direction. The sol–gel approach followed by first spinodal decomposition and later binodal decomposition phase separation has been taken in this study to initiate network formation throughout the P-VTMS/TPU backbone to form a 3-D network porous structure. The structure obtained offers full hierarchical multimodal porosity and an unprecedentedly large surface area of 2146 m2 g–1 due to a special approach taken in the sol–gel process in designing the interface between electrospun TPU nanofibers and P-VTMS chains. Owing to the combination of excellent mechanical and thermal insulation properties, the P-VTMS/TPU composite aerogel can be used as a thermal insulation material. Such a hierarchical multimodal porous architecture opens the door to fabricating new 3-D multifunctional and mechanically durable nanocomposite aerogels for flexible devices.
Polymers are known as thermally insulated materials with reported effective thermal conductivity (Keff) in the range of 0.1 to 0.5 Wm−1 K−1. However, increasing demand for smaller and more powerful ...electronics has created the need for thermally conductive polymers for use in heat exchangers and electronic packaging applications. Given this background, much research has been done over the past two decades to increase the Keff of polymers. Based on the strategy involved, those works can be divided into two main categories: (i) increasing the Keff of the neat polymer by aligning its chains orientation; and (ii) increasing polymer Keff by fabrication of polymeric composites with thermally conductive filler networks. Among these two strategies, the former is limited to nanoscale laboratory research and is difficult to scale up for mass production. Therefore, this work is mainly focused on the latter category, thermally conductive polymeric composites, which has a higher potential for large‐scale production. This work aims to summarize, evaluate, and highlight the successful strategies of the recent efforts in enhancing the thermal conductivity of polymer composites. The major achievements, future challenges, and the outlooks of high thermally conductive polymeric composites are presented by analyzing the results of about 300 works.
Polyimide (PI) aerogels are promising in various fields of application, ranging from thermal insulators to aerospace. However, they are typically in the form of a bulk monolith, which suffers from a ...lack of conformability and drapability. Moreover, their electrical conductivity is limited, and they mainly display an insulative behavior. These shortcomings can limit the applications of PI aerogels in energy storage systems, which require ultralightweight flexible conductive films, which at the same time offer high thermal stability, ultralow density, and high surface area. To overcome these obstacles, the present study reports the fabrication of PI-carbon nanotube (PI-CNT) aerogel composite films with varying CNT content prepared through a sol–gel preparation method, followed by a supercritical drying procedure. Compared to pristine PI aerogels, which displayed a large shrinkage and density of 18.3% and 0.12 g cm–3, respectively, the incorporation of only 5 wt % CNTs resulted in a significant reduction of both shrinkage and density to only 11.5% and 0.10 g cm–3, respectively. This suggests the importance of CNTs in improving the dimensional stability of aerogels and creating a robust network. Further characterizations showed that incorporation of 5 wt % CNTs also resulted in the highest pore volume (1.25 cm3 g–1), highest surface area (324 m2 g–1), highest real permittivity (80), highest electrical conductivity (3 × 10–1 S m–1), and ultrahigh service temperature (575 °C). It was also shown that the aerogel films can withstand a large degree of bending, can be twisted, and can be fully rolled with no obvious cracks propagated in the structure. The combined outstanding properties of the developed aerogel composite films make them promising potential candidates for supercapacitor electrodes. Therefore, the electrochemical performance of the devices based on aerogel electrodes was further studied. The device demonstrated a high energy density of 2.6 Wh kg–1 at a power density of 303.8 W kg–1. The total capacitance after 5000 cycles was 91.8% of the initial capacitance, which indicated excellent stability and durability of the device. Overall, this work provides a facile yet effective methodology for the development of high-performance aerogel materials for energy storage applications.
In heat transfer analysis of AFP process using a hot gas torch, the convective heat transfer which occurs between the hot gas flow generated by a torch nozzle and a composite substrate plays an ...important role in the heat transfer mechanism. In order to model the convective heat transfer, a local heat flux equation
is utilized where
is the energy flow per unit of area per unit of time, h is the convective heat transfer coefficient between the hot gas torch and the composite surface, and
accounts for the temperature difference between the two media. This coefficient h is dependent on various number of parameters such as nozzle geometry and its configuration relative to the surface of the substrate, type and configuration of the roller, gas flow rate, temperature of the gas, type of the gas etc. Researchers on the heat transfer analysis for automated composites manufacturing have used values of h that vary from 80 W/m
2
K to 2500 W/m
2
K. This large range gives rise to uncertainties in the determination of important behavior such as the temperature distributions, residual stresses, and deformations of the composite structures due to the manufacturing process. The reason for these large differences can be due to the differences in the process parameters in each of the studies. The process parameters can include the volume flow rate of the hot gas, the gas temperature, the distance between the nozzle exit and the surface of the composite plate, the angle of the torch with respect to the surface of the substrate etc. In addition, the value of the h coefficient may not be constant over the heating length of the process. The purpose of this paper is three fold: 1. To investigate the AFP process parameters that may affect h. 2. To investigate different methods for the determination of h, and 3. To develop a procedure for less-time-consuming determination of h for the purpose of analysis for residual stresses and deformations.
Data‐driven modeling in material science rose to prominence in the last decade, and various supervised and unsupervised machine learning techniques have been employed for material development and ...deriving insights for decision‐making purposes. In this context, machine learning can have prominent importance in the field of nanostructured aerogels for accelerated materials design and material properties prediction. Current attempts rely only on experimental approach, which have inherent shortcomings, including inefficiency due to the prolonged synthesis process, and necessity of analyzing microstructure and properties. In order to address the challenges associated with the traditional experimental approach, in this study, an artificial neural network (ANN) is employed to predict the material properties of nanostructured aerogels. Polyimide (PI) organic aerogels are selected for this purpose. Through understanding the contributing material and processing factors in PI aerogel synthesis, a dataset is prepared. Data preprocessing is performed, and through hyperparameter tuning, ANN is constructed and trained for a given dataset. Various material properties are predicted, including compressive modulus, density, and porosity. Results show that ANN is trained with high accuracy, which demonstrates the versatility and accuracy of model in materials properties prediction. This study can therefore pave the way for establishing a platform for data‐driven materials innovation.
This study focused on the development of a machine learning predictive model as a new toolbox to mitigate costs, risks, and time associated with the traditional experimental approach in aerogel materials fabrication. To achieve this objective, an artificial neural network model was constructed with optimized topology. Various properties of aerogels including compressive modulus, density, and porosity were accurately predicted and a procedure for the use of model for novel aerogel materials development is recommended.
With more and more use of composites in engineering applications, the need for automated composites manufacturing is evident. The use of automated fiber placement (AFP) machine for the manufacturing ...of thermoplastic composites is under rapid development. In this technique, a moving heat source (hot gas torch, laser, or heat lamp) is melting the thermoplastic composite tape and consolidation occurs in situ. Due to the rapid heating and cooling of the material, there are many issues to be addressed. First is the development of the temperature distribution in different directions which gives rise to temperature gradients. Second is the quality of the bond between different layers, and third is the rate of material deposition to satisfy industrial demand. This paper addresses the first issue. The temperature distribution affects the variation in crystallinity, and residual stresses throughout the structure as it is being built. The result is the distortion of the composite laminate even during the process. In order to address this problem, first the temperature distribution due to a moving heat source needs to be determined. From the temperature distribution, the development and distribution of crystallinity, residual stresses and deformation of the structure can then be determined. As the first phase of the work, this paper investigates the temperature distribution due to a moving heat source for thermoplastic composites, without considering the material deposition. A finite difference (FD) code based on energy balance approach is developed to predict the temperature distribution during the process. Unidirectional composite strips are manufactured using AFP and fast-response K-type thermocouples (response time of 0.08 s, as compared to normal thermocouples with response time of 0.5 s) are used to determine the thermal profiles in various locations through the thickness of the composite laminate subjected to a moving heat source. It is shown that temperature variations measured experimentally during the heating pass, using thermocouples embedded into the composite substrate, underneath layers of the composite material, are consistent with the generated thermal profiles from the numerical model. The temperature distribution, in both the direction of the tape and through-thickness direction can be predicted numerically.
Since the introduction of triboelectric nanogenerators (TENGs), a variety of dielectric and electrode materials have been developed to improve the TENGs output performance. However, due to their ...inherent shortcomings, including mechanical deficiencies, insufficient contact area, and limited breathability, the development of TENGs with diverse features and multi functionalities has been limited. To address these challenges, various porous materials and structural designs have been explored in the last decade for development of dielectrics and electrodes. Porous materials include hydrogels, aerogels, foams, and fibrous media, which due to their unique interconnected porous network, high surface area, and tunable pore size distribution, have been shown to be effective in improving the performance and versatility of TENGs. Alternatively, structural designs have also been introduced in the form of textiles and yarns to further expand the functionalities of TENGs. Compared to the first approach, such designs provide larger pores, which can be implemented as air gap in TENG designs. Therefore, these can provide the opportunity for the fabrication of all-in-one TENGs. Given the importance of the above strategies in improving the performance of TENGs and expanding their functionalities, this work aims to summarize, categorize, and highlight the recent advancements in the field of porous materials and structural designs. Representative applications of porous TENGs, including human-machine interfaces, smart wearables, and self-powered sensors with respect to the pore size, are reviewed. The current challenges and future perspectives for integrating porous materials and structures into TENGs are also discussed.
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•Porous materials and structures are studied for the TENG dielectrics and electrodes.•Porous materials include hydrogels, aerogels, foams, and fibrous media.•Porous structure assembly includes textile-based and yarn designs.•Pore size of the porous TENG systems varies in the range of nano to macro meter.•The benefits and challenges of porous systems for the TENG application are reported.
Due to their high service temperature, excellent thermal insulation, nanoporous morphology, and low dielectric constant, polyimide (PI) aerogels have the potential capability to be used in the next ...generation of microelectronic devices and flexible electronics. The main challenge, however, is controlling the shrinkage during the sol-gel process, and developing thin films with improved thermal insulation, reduced shrinkage and density, and controllable thickness while maintaining mechanical flexibility. Yet, the majority of previously reported PI aerogels were in the form of bulk monolithic, with a very limited number of studies on other configurations. In this study, thin film PI aerogels are developed
via
a doctor blade applicator. Through control of processing parameters, namely solution viscosity, casting speed and inter-blade spacing, thin film PI aerogels with controllable thickness and uniformity are fabricated. The PI aerogel network is formed through imidization of flexible 4,4′-oxydianiline (ODA) and biphenyl-tetracarboxylic acid dianhydride (BPDA) monomers. Replacing 50 mol% of the ODA with a more rigid 2,2′-dimethylbenzidine (DMBZ) in the oligomer backbone gives aerogels with improved strength, enhanced hydrophobicity and lower dielectric constant. 1,3,5-Benzenetricarbonyl trichloride (BTC) is used to create a cross-linked network with strong integrity. Compared to previously used cross-linkers (TAB, OAPS, or TAPP), BTC has a lower cost and is commercially available. Several characterization techniques were performed to examine the physical, chemical, thermal, mechanical, electrical, and optical properties of the fabricated aerogels. These highlighted the multifunctionality of the PI aerogels, combining outstanding mechanical flexibility (fully bendable and rollable), excellent strength (compression
E
= ∼1.9 MPa), ultralow thermal conductivity (27.5 mW m
−1
), lightweight (
ρ
= 0.089 g cm
−3
), superior service temperature (over 560 °C), ultralow dielectric constant (∼2.7), and excellent self-extinguishing behavior. As a potential application of the fabricated thin film PI aerogels, microelectronics packaging can be thus proposed.
Due to their high service temperature, excellent thermal insulation, and nanoporous morphology, polyimide (PI) aerogels have the potential capability to be used in the next generation of microelectronic devices and flexible electronics.
In this study, we utilized in situ nanofibrillation of thermoplastic polyester ether elastomer (TPEE) within a high-density polyethylene (HDPE) matrix to enhance the rheological properties, ...foamability, and mechanical characteristics of the HDPE nanocomposite at both room and subzero temperatures. Due to the inherent polarity differences between these two components, TPEE is thermodynamically incompatible with the nonpolar HDPE. To address this compatibility issue, we employed a compatibilizer, styrene/ethylene-butylene/styrene copolymer-grafted maleic anhydride (SEBS-g-MA), to reduce the interfacial tension between the two blend components. In the initial step, we prepared a 10% masterbatch of HDPE/TPEE with and without the compatibilizer using a twin-screw extruder. Subsequently, we processed the 10% masterbatch further through spun bonding to create fiber-in-fiber composites. Scanning electron microscopy (SEM) analysis revealed a significant reduction in the spherical size of HDPE/TPEE particles following the inclusion of SEBS-g-MA, as well as a much smaller TPEE nanofiber size (approximately 60–70 nm for 5% TPEE). Moreover, extensional rheological testing revealed a notable enhancement in extensional rheological properties, with strain-hardening behavior being more pronounced in the compatibilized nanofibrillar composites compared to the noncompatibilized ones. SEM images of the foam structures depicted substantial improvement in the foamability of HDPE in terms of the cell size and density following the nanofibrillation process and the use of the compatibilizer. Ultimately, the in situ rubber fibrillation and enhancement of HDPE and TPEE interface using a compatibilizer led to increasing the HDPE ductility at room and subzero temperatures while maintaining its stiffness.
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