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•Highly flexible CNTs based monolithic support was constructed via a directional freezing method.•Monolithic CNTs based composite PCM exhibits superior mechanical and thermal ...properties.•Flexible mechanism of monolithic CNTs based composite PCM is proposed.•This design strategy provides a new direction for future wearable fitting-skin temperature-controlled materials.
Currently, most reported composite phase change materials (PCMs) are powdery shape, which require secondary processing for practical applications. Although some monolithic composite PCMs have been developed, their flexibility usually undergoes a remarkable reduction or even complete disappearance when supporting materials are infiltrated with PCMs. To solve this problem, we fabricated a flexible supporting material with a folded layer-bridge network structure by dispersing carbon nanotubes (CNTs) in acetic acid solution of chitosan (CS) with poly(vinyl alcohol) (PVA) using a directional freezing method. Then CS/PVA/CNTs (CPC) scaffold was infiltrated with polyethylene glycol (PEG) to prepare PEG@CPC composite PCM. The resulting flexible composite PCM displays excellent mechanical properties, such as high tensile strength of 2.42 MPa and bending resistance of >100 cycles. Moreover, it displays outstanding thermal properties, such as high crystallinity of close to 100% and encapsulation ratio of 92.6 wt%. This work provides a simple method for preparation of flexible monolithic composite PCMs for many potential applications, such as wearable fitting-skin temperature-controlled materials.
Phase change materials (PCMs) applied in the energy storage and temperature control system are crucial for energy conservation and environmental protection. In this work, boron nitride (BN)@chitosan ...(CS) scaffolds with three-dimensional (3D) porous structures were fabricated. And effective thermal conductive pathways could be created in the resultant scaffolds. By introducing polyethylene glycol (PEG) into the BN@CS scaffolds, composite PCMs with large latent heat of fusion and excellent shape-stability were obtained. In particular, a high thermal conductivity up to 2.77 W m−1 K−1 could be reached at a relatively low content of BN (27 wt%). Moreover, they also exhibited a satisfactory energy storage density of 136 J g−1. This work demonstrated a facile and environmentally friendly strategy to simultaneously achieve enhancement of thermal conductivity, high energy storage density, shape stability and outstanding thermal repeatability for composite PCMs, which held promising potential in waste heat recovery, cooling system and temperature control system.
The propensity for various high-temperature shape memory alloys (HTSMA), i.e., Ni28.5Ti50.5Pt21, Ni24.5Ti50.5Pd25 and Ni24.5Ti50Pd25Sc0.5, to exhibit two-way shape memory effect (TWSME) was compared ...to that of a conventional binary Ni49.9Ti50.1 shape memory alloy (SMA). Thermomechanical training in the form of thermal cycling under constant stress levels was employed to induce two-way shape memory behavior in the various materials. The resulting TWSME was characterized for its magnitude and stability under stress-free conditions, while parameters such as training stress and upper cycle temperature during training were investigated for their influence on this phenomenon. For Ni49.9Ti50.1, a negative correlation was found between an increasing training stress, from 80MPa to 200MPa, and the magnitude of the resulting TWSM strain, while a positive correlation was observed for Ni24.5Ti50.5Pd25 and Ni24.5Ti50Pd25Sc0.5. The stability of the TWSME for the Ni49.9Ti50.1, measured by the strain evolution of the cold (martensitic) and hot (austenitic) shapes of the samples upon stress-free thermal cycling, was found to depend on the stress and temperature interval during training. Conversely, the stability of the NiTiPd based HTSMAs was much greater and less sensitive to these parameters over the stress and temperature intervals investigated. No TWSME was seen in Ni28.5Ti50.5Pt21 due to the higher upper cycle temperatures required during thermal cycling, which resulted in the recovery of any favorable dislocation structures generated during training.
Effect of copper–water nanofluid as a cooling medium has been studied to simulate the behavior of heat transfer due to laminar natural convection in a differentially heated square cavity. The ...transport equations for a non-Newtonian fluid have been solved numerically following finite volume approach using SIMPLER algorithm. The shear stresses have calculated using Ostwald–de Waele model for an incompressible non-Newtonian fluid. The thermal conductivity of the nanofluid has been calculated from the proposed model by Patel et al. Study has been conducted for Rayleigh number (
Ra) 10
4 to 10
7 while solid volume fraction
(
ϕ
)
of copper particles in water varied from 0.05% to 5%. It has been observed that the heat transfer decreases with increase in
ϕ for a particular
Ra, while it increases with
Ra for a particular
ϕ. The copper nanoparticle diameter has been taken as 100 nm for all of the studies.
Confronted with limited energy and material resources and undesirable manmade climate changes, science is searching for new and innovative strategies to save, transfer and store thermal energy. ...Currently, one of the most intensively discussed options are the so-called nanofluids. Nanofluids are suspensions consisting of a liquid basefluid and solid particles of sizes ranging from 10 nm to 200 nm. The higher thermal conductivity of these nanoparticles leads to an increased effective thermal conductivity of the fluid which, the general expectation is, should enhance the heat transfer of the device.
This overview aims to compile results of the application of nanofluids in thermosyphons, heat pipes, and oscillating heat pipes. The general goal is to draw conclusions with respect to the potentials for improvement of the thermal performance of these gadgets. Additionally, possible mechanisms which may generate these improvements are discussed. All together 38 experimental studies and 4 modelling approaches are analyzed. While most investigations recognize nanofluids as an advantageous working fluid, some others report negative effects.
Performance effects which are related to filling ratio, inclination angle, and operation temperature seem to be similar to those for classical working fluids. Several authors report a decrease of the thermal resistance or an increase of the efficiency with increasing concentration, but also a reversing of this trend if a certain optimal concentration is exceeded. This observation mainly follows with a significant increase of the evaporator heat transfer coefficient. The condenser heat transfer coefficient seems to be not or only weakly affected. Basefluid, nanoparticle material, size and shape, and the stabilization of the suspension have an influence on the thermal performance. However, the limited number of experiments does not allow drawing firm conclusions. The main mechanism responsible for the improved thermal performance seems to be a porous layer built from nanoparticles on the evaporator surface. Additional positive effects may follow from the changed thermophysical properties of the working fluid.
•The current research on thermosyphons, heat pipes and oscillating heat pipes operated with nanofluids is analyzed.•Effects with respect to gadget and nanofluid parameters and the thermal performance of gadgets are discussed.•A part of the analysis is dedicated to mathematical modelling.•Possible mechanisms which could explain the observed enhancement or deterioration of thermal performance are discussed.•Eleven clear conclusions are drawn from the analysis which indicate new directions of further research.
Recently, colorless and transparent plastic substrates with high glass transition temperature and suitable thermo‐mechanical properties are in high demand in optoelectronic device industries, due to ...their various optical applications. Colorless polyimides (CPIs) exhibit both high thermo‐mechanical stability as well as high transparency that are suitable for optoelectronic applications. The most recent (2010–2018) developments on CPIs, in terms of design and synthesis of new monomers and their effects on the thermo‐optical properties of the obtained CPIs, are reviewed in this article.
This article overviews colorless and transparent polyimides (CPIs) that can be used for plastic substrates with high glass transition temperatures and suitable thermo‐mechanical properties in optoelectronic device industries. The most recent (2010–2018) development on CPIs in terms of design and synthesis of new monomers and their effects on the thermo‐optical properties of the obtained CPIs are reviewed.
The authors explain the changes in the thermophysical and thermomechanical properties of polymer composites under elevated temperatures and fire conditions. Using microscale physical and chemical ...concepts they allow researchers to find reliable solutions to their engineering needs on the macroscale. In a unique combination of experimental results and quantitative models, a framework is developed to realistically predict the behavior of a variety of polymer composite materials over a wide range of thermal and mechanical loads. In addition, the authors treat extreme fire scenarios up to more than 1000?C for two hours, presenting heat-protection methods to improve the fire resistance of composite materials and full-scale structural members, and discuss their performance after fire exposure. Thanks to the microscopic approach, the developed models are valid for a variety of polymer composites and structural members, making this work applicable to a wide audience, including materials scientists, polymer chemists, engineering scientists in industry, civil engineers, mechanical engineers, and those working in the industry of civil infrastructure.
Miniaturization and high frequency are two key features of the next generation electronic devices, thus advanced electronic packaging materials with high thermal conductivity and excellent ...electromagnetic shielding performance (EMI SE) are highly expected. This work presents poly (vinylidene fluoride)@multi-wall carbon nanotube/boron nitride (PVDF@MWCNT/BN) composites with effective EMI SE and satisfactory insulation and thermal conductivity through a tailor-made segregated double network. PVDF@MWCNT composite microspheres were fabricated as the thermal and electrical conductive micro-network, then another BN macro-network was further prepared to provide electrical insulation and further thermal conductivity enhancement. This so-called double segregated network of PVDF@MWCNT/BN composites shows a thermal conductivity of 0.83 W m−1 K−1, electron insulation of 8.33 × 10−14 S cm−1 and electromagnetic shielding of 8.68 dB at 8.2 GHz when loaded 5 wt% MWCNT and 40 wt% BN. The study sheds new lights on the development of special functional materials and the PVDF@MWCNT/BN composites are highly promising as a future electronic packaging material.
