Low Thermal Conductivity Values Research Articles

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.

Insights into the physical properties and anisotropic nature of ErPdBi with an appearance of low minimum thermal conductivity

We theoretically study the structural, elastic and optical properties of ErPdBi together with its anisotropic behaviors using density functional theory. It is observed that ErPdBi satisfies the Born stability criteria nicely and possesses high quality of machinability. The anisotropic behavior of ErPdBi is reported with the help of theoretical anisotropy indices incorporating 3D graphical presentation, which suggests that ErPdBi is highly anisotropic in nature. It is noticed that the minimum thermal conductivity is very low for ErPdBi compared to the several species. This low value of minimum thermal conductivity introduces the potentiality of ErPdBi in high-temperature applications such as thermal barrier coatings. In addition, deep optical insights of ErPdBi reveal that our material can be used in different optoelectronic and electronic device applications ranging from organic light-emitting diodes, solar panel efficiency, waveguides etc. to integration of integrated circuits. Therefore, we believe that our results will provide a new insight into high-temperature applications and will benefit for the development of promising optoelectric devices as well.

Dust filter of secondary aluminium industry as raw material of geopolymer foams

In this work, the use of waste dust filter of secondary aluminum industry (DFA) to obtain geopolymer foams has been studied. The waste was used as source of alumina and foaming agent. As precursor and principal reactive silica supplier rice husk ash was used. Precursors were chemically activated by means of a sodium hydroxide aqueous solution and a commercial sodium silicate solution. The influence of the DFA content or Si/Al molar ratio (4–7) were determined by keeping the Si/Na molar ratio of 0.7 M constant and the concentration of sodium hydroxide in the activating solution equal to 8.5 M. The geopolymer foams obtained were studied by X-ray Diffraction (XRD), adsorption/desorption of nitrogen, infrared spectroscopy (FTIR), and scanning electron microscope (SEM) techniques. The results indicated that geopolymer foams presented low values of bulk density (643–737 kg/m3) high values of apparent porosity (62–70%), low, but sufficient values of compressive strength (0.5–1.7 MPa) and good values of thermal conductivity (0.131–0.157 W/mK). Lower values of thermal conductivity were obtained for Si/Al = 4 and 5 M ratios, due to the highest apparent porosity and the highest total pore volume. These geopolymer foam materials have similar properties to other construction materials sector such as gypsum boards, foamed concrete, or insulating materials. In addition, its use in other applications of interest such as catalyst support or gas filtration materials could be investigated.

Synthesis and Characterisation of Acrylic Resin-Al Powder Composites Suitable for Additive Manufacturing.

Stereolithography is an additive manufacturing technology commonly used to build either prototypes or final parts. Nevertheless, the manufacture of structural parts has been ruled out owing to the poor mechanical properties of conventional UV-curable resins. Moreover, the inventory of available commercial resins is still limited and they exhibit low thermal and electrical conductivity values. In this work, some composite materials were designed using Al microparticles dispersed within an SLA commercial resin matrix. These composites overcame the difficulties caused by the light scattering effect during the photopolymerisation process in the SLA technology. Dispersion of the filler was characterised by means of SEM/EDX and AFM. The composites exhibited improved thermal and mechanical behaviour in comparison with the pristine resin. The simplicity of the synthesis method used to prepare the composites provides a convenient starting point to explore new ways of designing composites for SLA with improved mechanical and functional properties.

Thermoelectric Transport Properties of n-Type Sb-doped (Hf,Zr,Ti)NiSn Half-Heusler Alloys Prepared by Temperature-Regulated Melt Spinning and Spark Plasma Sintering

Herein we report a significantly reduced lattice thermal conductivity of Sb-doped Hf0.35Zr0.35Ti0.3NiSn half-Heusler alloys with sub-micron grains (grain size of ~300 nm). Polycrystalline bulks of Hf0.35Zr0.35Ti0.3NiSn1−xSbx (x = 0.01, 0.02, 0.03) with a complete single half-Heusler phase are prepared using temperature-regulated melt spinning and subsequent spark plasma sintering without a long annealing process. In these submicron-grained bulks, a very low lattice thermal conductivity value of ~2.4 W m−1 K−1 is obtained at 300 K due to the intensified phonon scatterings by highly dense grain boundaries and point-defects (Zr and Ti substituted at Hf-sites). A maximum thermoelectric figure of merit, zT, of 0.5 at 800 K is obtained in Hf0.35Zr0.35Ti0.3NiSn0.99Sb0.01.

The effect of particle size, temperature and residence time on the yields and reactivity of olive stones from torrefaction

Abstract Olive stones obtained as a by-product from olive oil extraction in combination with the favourable climate in Mediterranean countries are value-added feedstocks for the energy sector due to low moisture content ( 20 wt. %), suitable calorific value ( > 18.7 MJ kg−1 as received) and high bulk density (about 750 kg m−3). The torrefaction process at Arigna Fuels with high energy efficiency of (above 90%) improves biomass properties for conversion to a high-value fuel for use in solid fuel stoves. This study reports the effect of moisture content, organic composition, inorganic matter, particle size, heat treatment temperature and residence time on product yields, O2/CO2 reactivity, calorific value, composition and thermal conductivity value of torrefied olive stones. Results showed that both lignocellulosic content and ash composition equally influenced the reactivity of torrefied material. For the first time, time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that the structure of torrefied material from small olive stone particles contains more cellulose than lignin when compared to large grains. Importantly from a technological standpoint, the lower heating values of torrefied olive stones (21.8 MJ kg−1) [1] from a small scale reactor were within the range of values for torrefied woodchip briquettes containing high starch binder content which was an energy increase of ≈ 15% when compared to the raw feedstock. The results showed that olive stones of particle size ≤ 2 mm produced during torrefaction at 270 °C for 30 min are the most suitable material and conditions for briquetting due to high solid yield, low reactivity and low thermal conductivity values. These conditions are recommended for the pilot plant operation using olive stones from the Mediterranean region.

Influence of effective thermal conductivity on hydrogen sorption in Mg-LaNi4.6Al0.4 composite hydride beds for thermal energy storage

Thermochemical energy storage using metal hydrides is well known. Appreciable quantities of heat energy interactions involved in the hydrogenation and dehydrogenation processes of metal hydride make the metal hydrides suitable for the storage of thermal energy. This paper discusses the synthesis, Pressure Concentration Isotherm (PCI) characterization and estimation of thermal properties of Mg + 50 wt% LaNi4.6Al0.4 composite hydride suitable for thermal energy storage. Maximum theoretical energy density of 918 kJ/kg can be achieved in the temperature range of 150–200 °C. Low values (0.1–1 W/mK) of effective thermal conductivity (ETC) of metal hydride beds is responsible for poor heat and mass transfer characteristics of metal hydride beds. The ETC of Mg + 50 wt% LaNi4.6Al0.4 composite hydride bed was augmented by fabricating two types of pellets. The sorption kinetic measurements were performed for three different configurations of metal hydride beds, one with powder composite hydride and remaining with two different types of pellets. Enhancement in ETC improved the heat and mass transfer characteristics of the composite metal hydride bed.

Ultra-low thermal conductivity of orthorhombic CH3NH3SnI3: A first principles investigation

Here, we report the first principles studies of the mechanical, dynamical, electronic and thermal conductivity in the orthorhombic phase of CH3NH3SnI3, which belong to perovskite family. The mechanical and dynamical properties ensure the stability of the investigated system. The electronic structure properties reveal the semiconducting nature with narrow band gap of 0.5 ​eV. The detailed analysis of other phonon based properties like phonon lifetime, mean free path along with phonon spectrum reveal the enhanced phonon-phonon interactions, which are responsible for ultra low value of lattice thermal conductivity, which is the main highlights of the present work.

A density functional theory study of the thermoelectric properties of K3AuO

We report for the first time the structural, electronic, lattice dynamics, mechanical and electronic transport properties of K3AuO calculated using first-principles concepts within the framework of density functional theory. Electronic transport coefficents are derived at the level of the Boltzmann’s transport theory with the electronic relaxation times of K3AuO estimated using the deformation potential method. The calculated lattice constants and equilibrium volumes of K3AuO are in excellent agreement with the available experimental data. Moreover, the obtained elastic constants satisfy the elastic stability criteria and thus infer that K3AuO is mechanically stable. The calculated band gap of K3AuO is in good agreement with the experimental measured value. Furthermore, our obtained phonon spectrum do not have negative frequencies and thus indicate that K3AuO is dynamically stable. Our results also show that K3AuO possesses a low lattice thermal conductivity (κL) value of 0.54 W/mK at 300 K, a value which is even lower than the measured κL of Bi2Te3 (1.28 W/mK at 300 K). Moreover, p-doped K3AuO is found to produce a power factor which is 1 order of magnitude larger than that of practical Bi2Te3 at 300 K. A high thermoelectric figure of merit is revealed in p-type K3AuO (ZT = 2.41 at 750 K) which indicate that K3AuO is a promising candidate for thermoelectric applications.

The use of materials based on Si3N4 in the composition of composites for the design of high-temperature thermoelectric generators

The possibility of reducing the thermal conductivity of silicon nitride as a basis of high-temperature electrical converters was investigated in the thesis. Also, the values of thermoelectric figure of merit and efficiency of thermoelectric current generator for the case of refractory oxygen-free composites were simulated. During the study, the dependence between the m coefficient, which determines the maximum possible efficiency of the thermoelectric generator and the ZT thermoelectric figure of merit, was determined. It was shown that the coefficient of thermal conductivity of the studied materials ranges from 1,2 to 4·106 m2/s and is characterized by a negative temperature coefficient over the entire temperature range. It was found that the thermal conductivity of Si3N4-based materials varies from 2,1 to 5,1 W/(m·K) depending on the type of sintering activator. The use of Al2O3 as an activator makes it possible to obtain a 25% lower thermal conductivity value comparing to materials with the addition of MgO. For the first time, it was proved that currently it is not possible to achieve an efficiency of 0,5hT in Si3N4-based materials used as a composite basis for high-temperature thermoelectric generators development.

Preparation of high‐strength lightweight alumina with plant‐derived pore using corn stalk as pore‐forming agent

AbstractTo improve the mechanical and thermal insulation properties of lightweight alumina, which was prepared by using pore‐forming agent from biological sources, the silica sol‐infiltrated corn stalk was utilized. Spring back and hygroscopicity of corn stalk powder as well as cold compressive strength, thermal conductivity, microstructure, and pore size distribution of lightweight alumina were characterized. The results indicate that impregnation of silica sol leads to different degrees of decrease in spring back height due to achieving better mesh by silica gel between the corn stalk powders, and then improves the formability, although at the same time the large number of hydrophilic groups results in an increase in hygroscopicity. Furthermore, sol impregnated pore‐forming agent optimizes the microstructure of the lightweight alumina pores. Lightweight alumina with a cold compressive strength up to 48.64 MPa was produced, and with the silica sol concentration of 3 wt%, lower thermal conductivity values at all test temperatures were obtained. Hence, the use of corn stalk impregnated with the appropriate concentration of silica sol as pore‐forming agent could enhance the mechanical and thermal insulation properties of lightweight alumina for the spheroidization of pore shape, randomization of pore distribution as well as miniaturization of pore size.

Flat Plate Type Solar Collector Performance Using Double Thermal Insulation

Indonesia is a country that has a huge potential of solar radiation energy since it is located in the equator. This causes Indonesia to have a tropical climate and receives almost the same emission of solar radiation throughout the year. National average solar energy potential is 16 MJ/day. This energy potential can be used as a source of thermal energy. To improve the energy potential, a research performance is done by using double thermal insulation solar collectors so that the energy can be received optimally. One alternative that is used is adding Styrofoam and Polyurethane which are installed on all sides of the collector except the upper side. Styrofoam and Polyurethane have 3 cm thickness and low thermal conductivity value therefore it is able to isolate the energy inside the solar collector. This experiment was done in a day with energy coming at the test reached 8,798,634 j/m2 with 69% efficiency level.

Hybrid Structures Made of Polyurethane/Graphene Nanocomposite Foams Embedded within Aluminum Open-Cell Foam

This paper focuses on the development of hybrid structures containing two different classes of porous materials, nanocomposite foams made of polyurethane combined with graphene-based materials, and aluminum open-cell foams (Al-OC). Prior to the hybrid structures preparation, the nanocomposite foam formulation was optimized. The optimization consisted of studying the effect of the addition of graphene oxide (GO) and graphene nanoplatelets (GNPs) at different loadings (1.0, 2.5 and 5.0 wt%) during the polyurethane foam (PUF) formation, and their effect on the final nanocomposite properties. Globally, the results showed enhanced mechanical, acoustic and fire-retardant properties of the PUF nanocomposites when compared with pristine PUF. In a later step, the hybrid structure was prepared by embedding the Al-OC foam with the optimized nanocomposite formulation (prepared with 2.5 wt% of GNPs (PUF/GNPs2.5)). The process of filling the pores of the Al-OC was successfully achieved, with the resulting hybrid structure retaining low thermal conductivity values, around 0.038 W∙m−1∙K−1, and presenting an improved sound absorption coefficient, especially for mid to high frequencies, with respect to the individual foams. Furthermore, the new hybrid structure also displayed better mechanical properties (the stress corresponding to 10% of deformation was improved in more than 10 and 1.3 times comparatively to PUF/GNPs2.5 and Al-OC, respectively).

Thermal Properties of the Very Low Thermal Conductivity Ternary Chalcogenide Cu4Bi4M9 (M = S, Se)

Temperature‐dependent thermal properties of phase‐pure polycrystalline ternary chalcogenides Cu4Bi4S9 and Cu4Bi4Se9 are reported. The structure and bonding in these materials result in very low thermal conductivity values (<0.8 W m−1 K−1 at room temperature) for both materials. The lattice contribution, Debye temperatures, and Sommerfeld coefficient are obtained from low‐temperature heat capacity data that also indicate very small electronic contributions to the heat capacity for these materials. This study aids in the identification of new nontoxic, earth‐abundant resistive ternary chalcogenide materials with low thermal conductivity for potential thermal barrier coating and rewriteable storage applications.