Low Thermal Conductivity Values Research Articles

Enhanced in‐plane and through‐plane thermal conductivity and mechanical properties of polyamide 4.6 composites loaded with hybrid carbon fiber, synthetic graphite and graphene

AbstractPolyamide 4.6 has excellent properties, such as high temperature resistance, crystallinity, fatigue resistance, melting temperature, excellent creep resistance, toughness, and good wear properties, but low thermal conductivity values. For this reason, its use in thermal applications has been limited. In this study, its use in thermal applications can be increased by increasing its thermal conductivity with carbon‐based fillers and reinforcements added to the PA46 structure. The aim of this study is to examine the carbon fiber (CF), synthetic graphite (SG) and graphene nanoplatelet (G) loading on the mechanical, thermal, physical, and electrical properties of polyamide 4.6, which are produced using co‐ rotating twin‐screw extrusion. Tensile and flexural strength of polyamide 4.6 increased with the addition of CF at all weight fractions. The highest electrical conductivity value was measured as 4.01 S/cm in PA46‐20CF‐20SG‐3G composite material. The highest in‐plane thermal conductivity achieved in this study at 20 wt% CF, 20 wt% synthetic graphite, and 5 wt% graphene loading was 20.43 W/mK. However, the highest through‐plane thermal conductivity value was obtained to be 4.19 W/mK at 20 wt% CF. Graphene and synthetic graphite are more efficient to increase the in‐plane thermal conductivity, while CF is more efficient to increase the through‐plane thermal conductivity.

Estimation of the through-plane thermal conductivity of polymeric ion-exchange membranes using finite element technique

The aim of this study is to calculate the through-plane thermal conductivity of commercial polymeric ion-exchange membranes. Different membranes were considered to study the influence of membrane properties on the thermal conductivity values. In particular we focused on reinforcement, ion exchange capacity and membrane density and thickness. For this purpose, we use a simple experimental setup and a numerical simulation to estimate the thermal conductivity from the experimental temperature profiles. Once the system is calibrated, the model includes as the only unknown parameter the membrane thermal conductivity. To validate the method, the thermal conductivity of the well-known Nafion membranes has been determined, a very good agreement was achieved in context from reliable literature values. The study also provides the thermal conductivity of other polymeric ion-exchange membranes with great potential in diverse applications under non-isothermal conditions. The calculated thermal conductivity for the different ion-exchange membranes is in the range from 0.04 Wm−1K−1 to 0.42 Wm−1K−1. The results show that the reinforcement leads to lower values of thermal conductivity whereas a higher density or heterogenous structure leads to higher thermal conductivity values. The approach presented here, combining experimental and simulation techniques, may provide a basis for confirming the effect of the polymeric ion-exchange membrane properties on the thermal conductivity and may shed light on the best choice for the electrolyte of membrane-based applications performance under non-isothermal conditions.

Experimental Study of the Thermal Behavior of a Watercress Planted Roofed Cubic Cell to be Watered with Domestic Wastewater

One of the virtues of watercress is its ability to grow in wastewater. This work aims at experimentally studying the thermal behavior of a watercress planted roofed cubic cell. To do this, the temperatures of various components of the cell and the solar radiation received by this cell were measured in order to compare the watercress roof performance with that of the conventional concrete roof. Then, the influence of the opening applied on the door of the studied cell was analyzed. As results, the fluctuation amplitude of the indoor ambient temperature of the concrete roofed cell is wider than that of the green roofed cell. Moreover, the last opening applied to the facades of the cell was the optimum area that the ambient temperature indoor was more attenuated. The LAI’s crop was worth 1.2. In addition, the low value of the canopy apparent thermal conductivity revealed that this layer plays a role of thermal insulation. The rooftop greening allows energy savings of about 85% compared to the consumed energy with conventional roofing. An extension of this work could be the energy performance analysis of a system using renewable energy for pumping domestic wastewater produced in or around green roofed housing.

Mechanical and thermal properties of glass/epoxy composites filled with silica aerogels

ABSTRACT This paper investigates the effect of silica aerogels on mechanical (flexural and impact), non-destructive and thermal properties of glass/epoxy composites. Silica aerogels were mixed with epoxy at various volume fractions (1% and 3%) and infused with glass fabrics to produce composite laminates. Flexural tests indicated that the addition of aerogels increased the flexural strength of composites at the warp direction. Low impact energy (10–30 J) test results exhibited that impact force and energy absorption of composites were slightly increased . Higher energy (100 J) caused more severe damages and the effect of aerogels was more obvious. The thermogravimetric analysis revealed that addition of aerogels increased the thermostability. Generally, addition 1% of aerogel provides more improvement on mechanical properties and thermal stability compared to addition 3%. Although there is not a linear relationship between aerogel content and thermal conductivity, samples with 3% aerogel exhibited lower thermal conductivity values than other samples.

Thermoelectric properties of Mo-doped bulk In2O3 and prediction of its maximum ZT

In a search for new thermoelectric materials, indium oxide (In2O3) was selected as a candidate for high-temperature thermoelectric oxide materials due to its intrinsically low thermal conductivity (<2 W/mK) and ZT values around 0.05. However, low electrical conductivity is a factor limiting the thermoelectric performance of this oxide, and was addressed in this study by Mo doping. It was found that Mo is soluble in In2O3 but forms secondary phases at a fraction near x = 0.06 and higher. Mo was found to be unsuitable for heavy n-type doping necessary to improve the thermoelectric performance of the oxide to the desired level (ZT = 1). However, the experimental data enabled us to analyze the electrical conductivity behavior and the Seebeck coefficient of doped In2O3 with different carrier concentrations, predicting a theoretically achievable maximum power factor value of 1.77 × 10−3 W/mK2 at an optimum carrier concentration. This estimation predicts the highest ZT value of 0.75 at 1073 K, assuming the lattice thermal conductivity value remaining at an amorphous level.

Low-Density Insulation Blocks and Hardboards from Amaranth (Amaranthus cruentus) Stems, a New Perspective for Building Applications

Nowadays, amaranth appears as a promising source of squalene of vegetable origin. Amaranth oil is indeed one of the most concentrated vegetable oils in squalene, i.e., up to 6% (w/w). This triterpene is highly appreciated in cosmetology, especially for the formulation of moisturizing creams. It is almost exclusively extracted from the liver of sharks, causing their overfishing. Thus, providing a squalene of renewable origin is a major challenge for the cosmetic industry. The amaranth plant has thus experienced renewed interest in recent years. In addition to the seeds, a stem is also produced during cultivation. Representing up to 80% (w/w) of the plant aerial part, it is composed of a ligneous fraction, the bark, on its periphery, and a pith in its middle. In this study, a fractionation process was developed to separate bark and pith. These two fractions were then used to produce renewable materials for building applications. On the one hand, the bark was used to produce hardboards, with the deoiled seeds acting as natural binder. Such boards are a viable alternative to commercial wood-based panels. On the other hand, the pith was transformed into cohesive and machinable low-density insulation blocks revealing a low thermal conductivity value.

Improvement in physico-mechanical and structural properties of rigid polyurethane foam composites by the addition of sugar beet pulp as a reactive filler

Green polymeric composites, which are obtained from cheap, renewable, and environmentally friendly natural resources that can replace petrochemicals, have gained great attention from the researchers in recent years. To that end, we aimed at preparing a type of bio-based rigid polyurethane foam (RPUF) composites which incorporated sugar beet pulp (SBP) particles as a reactive filler. The obtained composite foams were evaluated through the effect of the increasing amount of SBP particles on the microstructure, thermal and mechanical properties. To eliminate the density effect of foams on the physico-mechanical properties, the foams were obtained such that their densities were kept at 37 $$\pm$$ 0.5 kg.m−3. Besides, the heat transfer mechanism of foams in terms of radiative transfer, conduction through polymer matrix and gas phase were analyzed by using a predictive model for the efficient thermal conductivity. Based on FTIR results, the functional groups on SBP have a tremendous tendency to react with isocyanates in the presence of a catalyst. The introduction of 3 wt% SBP according to the total mass provided high compressive strength (166 kPa), low thermal conductivity value (20.46 mW/m.K) and excellent dimensional stability in harsh conditions. SEM images show that the distribution of cells was uniform and any broken cells were not detected in all composite foams. The addition of SBP particles generally decreases the radiative contributions and enhances the contribution of conduction through the polymer matrix. It is clear that for all foams, the conduction through gas-phase gave the biggest contribution to the total thermal conductivity. Thermogravimetric analysis data displayed that the inclusion of SBP in RPUF slightly improved the thermal stability of RPUF composites. All results indicate that SBP waste, which is the most produced by-agriproduct in sugar refining industry, is an advantageous natural filler in many aspects for preparing RPUF. The obtained environmentally friendly RPUF composites have all of the properties needed to meet the demand for thermal insulation engineering materials.

Comprehensive study on the physical properties of tetragonal LaTGe3 (T = Rh, Ir, or Pd) compounds: An ab initio investigation

This study uses density functional theory to investigate the structural, mechanical, electronic, optical, and thermodynamic properties of tetragonal LaRhGe3, LaIrGe3, and LaPdGe3 compounds. The investigated lattice parameter showed similar results to the experimental data, justifying the accuracy of our calculations. The negative values of formation enthalpy confirmed the thermodynamic stability of LaTGe3 (T = Rh, Ir, or Pd). The mechanical stability of these compounds was also verified by their single independent elastic constants. Poisson’s and Pugh’s ratios revealed that all the compounds have a ductile nature. The metallic nature of these phases was found from their band structure calculations. The study of Mulliken bond populations and charge density maps ensured the existence of a mixed character of ionic, covalent, and metallic nature in LaRhGe3, LaIrGe3, and LaPdGe3 compounds. Detailed investigation was also performed on optical properties, and the dielectric function, absorption, and conductivity again ensured the metallic feature of all these phases. The calculated optical functions suggested their potential application in quantum-dot light emitting diodes, organic light emitting diodes, solar cells, waveguides, and solar heating reduction. Moreover, the very low values of minimum thermal conductivity and the Debye temperature are indicative of their suitability for thermal barrier coating materials.

An uncomplicated method for growing nano-quasicrystalline structures in the AlCuFeB quaternary alloy system: A short-time milling

Quasicrystals have been comprehensively studied since their discovery in 1984 by Daniel Shechtman to dissolve their complex structures concerning their physical properties [1–5]. The quasi-periodic well-ordered atomic structures made QCs as a novel type of solids differing from the crystalline or amorphous materials [5–7]. The QC alloys have non-crystallographic rotational symmetries, such as five-fold, eight-fold, and even ten-fold rotational axes that are prohibited in ordinary periodic crystals [5–7]. The QC alloys have been recently considered for several applications owing to a series of the unusual combination of exclusive physicomechanical properties for metallic-based alloys [5–7]. Accordingly, the properties of these alloys can be pointed out to their high strength, elevated hardness, desirable corrosion and wear resistance, minute adhesion, low values of thermal and electrical conductivity, and superior optical properties [5–7].•This paper focuses on the synthesis, structural and microstructural evolutions, thermal stability, microhardness, and electrical and optical properties of the Al59Cu25.5Fe12.5B3 nanoquasicrystalline alloy for solar selective absorber usages. Accordingly, there are various advanced techniques for synthesizing the nanostructured quasicrystals, such as laser ablation, sol-gel, electros pining, mechanical alloying, and hydrothermal methods.•The structural and microstructural evolutions of the mechanically alloyed AlCuFeB powders were investigated by X-ray diffractometry and field-emission scanning electron microscopy. The thermal stability of the AlCuFeB powders was recorded by differential thermal analysis.•The nanostructured Al59Cu25.5Fe12.5B3 stable quasicrystalline phase was synthesized by short-time milling procedure in 1 h. It was found that the presence of the quasicrystalline phase in the AlCuFeB alloy prominently improves the microhardness, electrical resistivity, and sunlight absorptance.

The Effects of Polystyrene Species and Fiber Direction on Thermal Conductivity of Plywood

Thermal conductivity of wood material is superior to other building materials because of its porous structure. Thermal conductivity is used to estimate the ability of insulation of material. Thermal conductivity of wood material has varied according to wood species, direction of wood fiber, specific gravity, moisture content, resin type, and addictive members used in manufacture of wood composite panels. The aim of study was to determine the effect of polystyrene species and fiber direction on thermal conductivity of plywood panels. In the study, two different wood types (black pine and spruce), two different fiber directions (parallel and perpendicular to the plywood fiber direction), two different types of insulator (expanded polystyrene-EPS and extruded polystyrene-XPS) and phenol formaldehyde glue were used as the adhesive type. Thermal conductivity of panels was determined according to ASTM C 518 &amp;amp; ISO 8301. As a result of the study, the lowest thermal conductivity values were obtained in the perpendicular fiber direction of the spruce plywood using XPS as insulation material. The use of XPS as an insulation material in plywood has given lower thermal conductivity values than EPS.

Thermal Conductivity of Lightweight Concrete Block with Various Cooling Agent

Energy was the important sources to human life. Due to increases energy demand in daily life, the energy consumption was increase day by day because of the heat load from solar radiation and heat produced by people. Toward sustainable development, this research was carried out to develop a lightweight concrete (LWC) block with various cooling agent such as glycerine, propylene glycol, coconut shell and gypsum powder. Six lightweight concrete (LWC) block with the size 250mm (L) × 250mm (W) × 100mm (T) were tested for thermal conductivity value. From the experimental result, it shows that lightweight concrete (LCW) block with various cooling agent obtained thermal conductivity value of 0.17W/mK - 0.36W/mK lower than thermal conductivity value for normal lightweight concrete (0.8W/mK) depending on concrete density. The lightweight concrete (LCW) block with cooling agent having low thermal conductivity value will reduce energy consumption in building.

Solidification enhancement in triplex thermal energy storage system via triplets fins configuration and hybrid nanoparticles

Latent thermal energy storage dependent on Phase Change Materials (PCMs) proposes a possible answer for modifying the availability of alternating energy from renewable sources such as wind and solar. They can possibly store large amounts of energy in moderately tiny dimensions as well as through almost isothermal procedures. Notwithstanding, low thermal conductivity values is a significant disadvantage of the present PCMs which critically restrict their energy storage usage. Likewise, this unacceptably decreases the solidification/melting rates, hence causing the system response time to be excessively lengthy. The present examination accomplished a better PCM solidification rate with a combination of hybrid nanoparticle (MoS2 - Fe3O4) and novel fin configuration in triplex-tube storage. A computational model that considers the natural conduction was represented and validated against previous experimental data. The influences of applying various nanoparticle volume fractions, radiation parameter, and shape factor on the assessment of the liquid-solid interfaces, phase change rate, and solidification process time over the whole solidification procedure was calculated and reported. The outputs demonstrate that PCM solidification is becoming better by utilizing the aforementioned methods. The obtained results disclose that the radiation parameter has a significant impact on the phase change rate, which shows a 74.58% contribution to the full solidification process time (FST). Additionally, the optimum parameters have designed to optimize the full solidification process time in the triplex-tube latent heat energy storage system (LHESS) system by using the Taguchi and Response Surface Methodology (RSM) methods. As a novelty, an accurate correlation for FST is developed with sensibly great precision.

Synthesis, crystal structure and transport properties of the cluster compounds Tl2Mo15S19 and Ag3Tl2Mo15S19

We report the syntheses and crystal structures of the cluster compounds Tl2Mo15S19 and Ag3Tl2Mo15S19, two novel members of the series M2Mo15X19 (M = In, Tl, K, Rb, Cs, Ba; X = S, Se). While only single crystals of Tl2Mo15S19 were obtained, Ag3Tl2Mo15S19 powders were synthesized, allowing for a study of its thermoelectric properties. Their structures contain Mo6S8iS6a and Mo9S11iS6a units separated by Tl+ cations. Silver insertion increases the electron transfer towards the Mo clusters leading to modifications of the Mo-Mo distances. Electrical resistivity measurements on Tl2Mo15S19 evidence its metallic character. Inserting Ag drives the transport properties towards a semiconducting state characterized by relatively low electrical resistivity and thermopower values. The large, complex unit cell of these compounds results in very low lattice thermal conductivity values below 1 Wm−1 K−1. Although the Ag insertion improves the thermoelectric performances, the ZT value remains moderate with a maximum value of 0.2 at 800 K.

Thermal performance of heat and water recovery systems: Role of condensing heat exchanger material

A significant amount of sensible and latent heat can be recovered at low temperatures from the flue gas of process heating equipment. However, the condensation of acids and water vapor makes the flue gas a highly-corrosive environment, which is a challenge for condensing heat exchangers. Corrosion-resistant materials are usually expensive and/or have relatively low thermal conductivities. Hence, it is essential to characterize the role of thermal conductivity of heat exchanger material in the performance of condensing heat exchangers. In the present study, a new analytical model is proposed and validated against available experimental data to predict the thermal performance of a tube-bank heat and water recovery unit. Our study indicates a threshold for tube thermal conductivity (~0.75 ​W ​m −1 K −1 ), which is a point where further increase does not significantly improve the condensation efficiency. This relatively low value of thermal conductivity, compared to commonly-used materials, e.g., ~10–15 ​W ​m −1 K −1 for stainless steel, unlocks the potential of using materials such as natural graphite, plastics, polymers, and ceramics (with thermally conductive additives) for applications in condensing heat exchangers and heat/water recovery in industry.

Tribological characterization of epoxy hybrid nanocomposite coatings reinforced with graphene oxide and titania

Epoxies in the form of bulk and coatings have been used throughout the years for a wide spectrum of applications in industries. However, the application of epoxies in demanding tribological applications is often limited by the properties of the pristine epoxy matrix such as low load bearing capacity under severe p-v regimes combined with low thermal conductivity values. In this study, the load bearing capacity of the pristine epoxy coating system applied onto non-textured, plasma treated mild steel substrates was first evaluated under dry sliding conditions. Wear tests were conducted using a ball on disk configuration with a grade 440C hardened stainless steel ball with a diameter of 6.3 mm and a hardness of 62 HRC, as the counterface. Graphene oxide (GO) fillers in varying concentrations of (0.25, 0.5, 1 and 1.5 wt%) were added to the pristine epoxy matrix to produce Epoxy-GO nanocomposite epoxy coatings and tribological evaluation was carried out. Tribological tests revealed that the addition of GO nanoparticles at 0.5 wt% filler loading led to a significant improvement in wear life (~6.5 times) when compared to the pristine epoxy system. In the second phase of this study, titania (TiO2) fillers were added to the optimized epoxy-0.5 wt% GO nanocomposite coating to produce GO-TiO2 hybrid nanocomposite epoxy coatings and tribological evaluation was carried out. However, tribological tests did not indicate any significant increase in the wear life after the addition of TiO2 as compared to the Epoxy-GO nanocomposite epoxy coatings. A detailed study of coating failure and wear mechanisms at significant datapoints were carried out using a combination of techniques namely coefficient of friction graphs for wear life, SEM for wear track imaging, EDS for elemental analysis of the wear track to establish failure, optical profilometry and counterface imaging using optical microscopy.

A Pathway toward a New Era of Open-Cell Polyurethane Foams-Influence of Bio-Polyols Derived from Used Cooking Oil on Foams Properties.

In order to create greener polyurethane (PUR) foams, modified used cooking oils (UCO) were applied as starting resources for the synthesis of bio-polyols. The bio-polyols were produced using transesterification of UCO with diethylene glycol (UCO_DEG) and triethanolamine (UCO_TEA). Next, open-cell PUR foams were synthesized by replacing 20, 40, 60, 80 and 100% of the petrochemical polyol with the bio-polyol UCO_DEG or UCO_TEA. It was observed that an increasing bio-polyol content (up to 60%) led to an increase of the closed cell content. However, a further increase in the bio-polyol content up to 100% resulted in foam cell opening. The bio-foams obtained in the experiment had an apparent density of 13–18 kg/m3. The coefficient of thermal conductivity was determined at three different average temperatures: 10, 0 and −10 °C. The PUR bio-foams modified with bio-polyol UCO_TEA had lower values of thermal conductivity, regardless of the average temperature (35.99–39.57 mW/m·K) than the foams modified with bio-polyol UCO_DEG (36.95–43.78 mW/m·K). The compressive strength of most of the bio-foams was characterized by a higher value than the compressive strength of the reference material (without bio-polyol). Finally, it was observed that the bio-materials exhibited dimensional stability at 70 °C.

Probing local distortion around structural defects in half-Heusler thermoelectric NiZrSn alloy

The half-Heusler NiZrSn (NZS) alloy is particularly interesting owing to its excellent thermoelectric properties, mechanical strength, and oxidation resistance. However, the experimentally investigated thermal conductivity of half-Heusler NZS alloys shows discrepancies when compared to the theoretical predictions. This study investigates the crystal structure around atomic defects by comparing experimental and theoretical X-ray absorption fine structure (XAFS) spectra of the crystal structure of a half-Heusler NZS alloy. The results of both Zr and Ni K-edge XAFS spectra verified the existence of atomic defects at the vacancy sites distorting the C1b-type crystal structure. We concluded that the distortion of the atoms around the interstitial Ni disorder could be the probable reason for the observed lower thermal conductivity values compared to that predicted theoretically in half-Heusler alloys. Our study makes a significant contribution to the literature because the detailed investigation of the lattice distortion around atomic defects will pave the way to further reduce the thermal conductivity by controlling this distortion.

The performances of expanded graphite on the phase change materials composites for thermal energy storage

For the storage of latent thermal energy (LTES), phase change materials (PCM) are the most commonly used. Nonetheless, their low thermal conductivity values and the liquid leakage on the transition phase of process limits their application. Hence, the stabilization-form can be a solution to surmount these two limitations. In this work, the Hexadecane with large latent heat was used as PCM, styrene-b-(ethylene-co-butylene)-b-styrene (SEBS) tri-block copolymer and the low-density polyethylene (LDPE) served as the supporting materials and expanded graphite (EG) was added for improving the thermal conductivity. We focused on the preparation of SEBS/Hexadecane/LDPE Composites and the improvement of the heat transfer using the EG. The Fourier Transformation Infrared spectroscope also demonstrated a good compatibility between SEBS, LDPE, Hexadecane, and EG. The transient Guarded Hot Plate Technique (TGHPT) and a Thermogravimetric analyzer were utilized to assess the thermal properties and thermal stability of the PCM composites respectively. Further, a leakage test proved that the composite has an excellent form-stable property. Thanks to expanded graphite, no hexadecane leakage was depicted at a 75% mass fraction of PCM in composites, which surmounts almost all mass fraction values reported in the literature.

Observation of an Unidentified Phonon Peak in SiGe Alloys and Superlattices Using Molecular Dynamics Simulation

Molecular dynamics was applied to a variety of SiGe alloys and superlattice structures in order to investigate the production of an unidentified phonon mode. The mode in question, observed in the resulting phonon dispersion relations, was produced around 2.5 – 5 THz. The corresponding spectral energy density data for the SiGe alloys produced varying intensities around this region, with a higher intensity tending towards lower thermal conductivity values. The unidentified mode was produced by the alloy and the non-periodic superlattice structures, whereas in the case of the periodic superlattices, the mode is less conspicuous. Regarding the SiGe compound, the mode was not produced. It is concluded that the atomic disorder within the alloys and non-periodic superlattices induce a stronger intensity of the unidentified mode.

Cauchy type inverse problem in a two-layer area in the blades of gas turbine

In order to reduce the thermal load of the gas turbine blade, its surface is covered with an external ceramic layer of high thermal resistance. The main problem considered in our research is the selection of ceramics with a low value of specific thermal conductivity and its thickness of the layer. Obtained values should be chosen, such that the permissible metal temperature is not exceeded at the metal-ceramic boundary. It would result in the loss of mechanical properties. Therefore, for a given temperature distribution at the metal-ceramic boundary, the temperature on the inside of the blade and the initial temperature should determine the temperature function on the external surface of the ceramic. This is a Cauchy type problem. The paper considers the case of one-dimensional, unsteady flow. The influence of heat conduction ratio of the ceramic layer and thickness of the layer was investigated. The results showed, that the implementation of ceramic layer causes a decrease in temperaturee what improves the cooling of metal layer. The authors observed also that the thickness if ceramic layer is crucial – its value should be selected so that not to cause a loss of mechanical properties of metal layer.