High Thermal Conductivity Values Research Articles

A comprehensive experimental study regarding size dependence on thermal conductivity of graphene oxide nanosheet

Graphene-based materials have great application prospects in the thermal management field due to their high intrinsic thermal conductivity. Understanding the factors affecting the thermal conductivity of graphene and graphene-based materials is essential for further application of the materials. This study aims to explore the effect of graphene oxide (GO) nanosheet size on its intrinsic thermal conductivity experimentally. By simultaneously characterizing the morphology and thermal conductivity of the nanosheet, an obvious size dependence of thermal conductivity is observed: larger-sized GO nanosheets are preferred to have higher thermal conductivity values. Further detailed morphology and structural analysis identify that this dependence is caused by morphological defects. In addition, whether this dependence at the microscale still exists in practical GO-based materials is also studied. The characterization of thermal conductivity and thermal management performance for macroscopic materials confirms this inheritability. This work clarifies the dependence and its generation principle of thermal conductivity on the GO nanosheet size, meanwhile, it also proves that this dependence still exists when microscopic nanosheets are integrated into macroscopic systems.

Spark Plasma Sintering of SiAlON Ceramics Synthesized via Various Cations Charge Stabilizers and Their Effect on Thermal and Mechanical Characteristics

Oxygen-rich SiAlON ceramics doped with various nanosized metal oxide (MO) stabilizers were synthesized with a view to examine their effect on thermal and mechanical characteristics. The nanosized starting powder precursors comprising Si3N4, AlN, Al2O3, and SiO2 along with oxides of Ba, Y, Mg, La, Nd, Eu, Dy, Er, and Yb as the MO charge stabilizer were employed in developing different SiAlON samples. Ultrasonic probe sonication was utilized to develop a homogenous mixture of initial powder precursors followed by spark plasma sintering (SPS) of the samples at the low temperature of 1500 °C coupled with 30 min of isothermal treatment. Sample compositions (according to general formula of alpha SiAlON: Mm/vv+Si12−(m+n)Alm+nOnN16−n) selected in the present study are represented by m value of 1.1 and n value of 1.6. The synthesized samples were evaluated for their physical behavior, microstructural and crystal structure evolution, and thermal and mechanical characteristics. More specifically, the sintered ceramics were examined by X-ray diffraction and electron microscopy to comprehend and relate the structural characteristics with the densification, thermal conductivity, hardness, and fracture toughness. The high reactivity of the nanopowders and the localized heating provided by SPS resulted in densified ceramics with relative densities in the range of 92–96%. Vickers hardness values were found to be in the range of 12.4–17.0 GPa and were seen to be profoundly influenced by the grain size of the alpha SiAlON (primary) phase. The fracture toughness of the samples was measured to be in the range of 4.1–6.2 MPa·m1/2. SiAlON samples synthesized using Er and Yb charge stabilizers were found to have the highest fracture toughness of 5.7 and 6.2 MPa·m1/2, primarily due to the relatively higher content of the elongated beta phase. While there was no obvious relationship between the thermal conductivity and the alpha SiAlON metal charge stabilizers, the values were seen to be influenced by the grain size of alpha phase where Dy-SiAlON had the lowest thermal conductivity of 5.79 W/m⋅K and Er-SiAlON showed the highest value of thermal conductivity (6.91 W/m⋅K). It was concluded that scientifically selected metal oxide charge stabilizers are beneficial in developing SiAlON ceramics with properties tailored according to specific applications.

Thermophysical properties of water, water and ethylene glycol mixture-based nanodiamond + Fe3O4 hybrid nanofluids: An experimental assessment and application of data-driven approaches

This paper aims to study the thermophysical properties of water, water and ethylene glycol mixture-based nanodiamond + Fe3O4 hybrid nanofluids experimentally and numerically using two data-driven approaches, namely, Multivariate linear regression (MLR) and Multivariate linear regression with interaction (MLRI) models. Three types of base fluids, such as (i) water, (ii) 40:60% water and ethylene glycol mixture, and (iii) 60:40% water and ethylene glycol mixture, were used to prepare the hybrid nanofluids. For all the base fluid, ϕ is used as 0.05%, 0.1%, and 0.2%. Results indicate that, higher values of thermal conductivity and viscosity is 17.76% and 72.9% at ϕ = 0.2% and 60 °C in comparison to water data. Similarly, for 40:60% water and ethylene glycol mixturebased nanodiamond + Fe3O4 hybrid nanofluid, the maximum thermal conductivity and viscosity enhancements are 14.65% and 79.01% at ϕ = 0.2% and at 60 °C compared to the base fluid. However, the maximum thermal conductivity and viscosity enhancements for ϕ = 0.2% and at 60 °C of 60:40% water and ethylene glycol mixture-based nanodiamond + Fe3O4 hybrid nanofluid are 12.79% and 50.84% over the basefluid data. Empirical and data-driven correlations were proposed based on the experimental data. The developed data-driven models resulted in robust individual relationships to predict the thermophysical properties of all types of hybrid nanofluids by superior performance compared to corresponding empirical correlations. The reported results exhibited that for thermal conductivity, density, Specific heat, and viscosity estimation, the MLRI (R = 0.9996), MLR (R = 0.99989), MLR (R = 0.9999998), and MLRI (R = 0.9857) had the best predictive performance.

Thermo-mechanical properties of different structures of BC2N

ABSTRACT The thermo-mechanical properties of four various structures of BC2N are investigated using molecular dynamics (MD) simulation. These structures show superior mechanical properties. Especially, the type C, which has more C–C and B–N bonds, demonstrates higher mechanical properties than others. Conversely, the type D shows the lowest mechanical properties with increasing B–B and N–N bonds. BC2N structures are studied at different temperatures and strain rates. As temperature increases up to 900 K, the mechanical properties of BC2N structures gradually decrease. Also, changing of temperature shows more effect on the types A and D than others. Furthermore, the mechanical properties of BC2N structures demonstrate increasing trend when strain rate increases. Additionally, the influence of strain rate on the mechanical properties of BC2N structures is more prominent at lower temperatures. Non-equilibrium MD simulation findings show high thermal conductivity (TC) values of BC2N structures. The type C containing most C–C and B–N bonds has the highest TC value. However, the type D has more N–N and B–B bonds, resulting in a low TC. Due to increasing of phonon-phonon scattering, when the temperature increases from 300 to 900 K, the TCs of these structures are adversely affected.

Petrophysical Property Modifications by Alteration in a Volcanic Sequence at IODP Site U1513, Naturaliste Plateau

AbstractPetrophysical properties of volcanic rocks are controlled by lithology and subsequent modification by alteration processes. Investigating the linkages, using a range of different techniques, are important to establish how petrophysical properties can inform about the alteration state of volcanic rocks. Here, we compile petrophysical data and correlate these with geochemical and mineralogical analyses acquired from a volcanic sequence on the Naturaliste Plateau, offshore southwest Australia (International Ocean Discovery Program Site U1513). The sequence consists of alternating basalt lava flows and volcaniclastic deposits, intruded by multiple dolerite dikes. Variable alteration intensities from fresh‐slight to strong are quantified using Chemical Index of Alteration. Intervals of slightly altered dikes exhibit low porosity and high values of bulk density, P‐wave velocity, and thermal conductivity. The increase of alteration intensity corresponds to decreases in bulk density to ∼2 g/cm3, P‐wave velocity to ∼2,000 m/s, thermal conductivity to ∼1.2 W/(m·K) and an increase in porosity up to 50%. Natural Gamma Ray and magnetic susceptibility vary downhole with rock composition and at lithologic boundaries. The distinct variations exhibit a good correlation with primary lithologic characteristics and secondary mineralogical and textural changes attributed to alteration processes. We provide synthesis models of petrophysical variation with alteration intensity. Although differences in primary lithology and alteration type introduce limitations and uncertainties, there is a reasonable applicability of our results to rapidly characterize the alteration intensity and volcanic stratigraphy in volcanic sequences and to calibrate wireline log‐based determinations. This will help others to develop strategies for exploration, drilling, and geophysical research of volcanic rocks.

Numerical Analysis of Minimum Ground Temperature for Heat Extraction in Horizontal Ground Heat Exchangers

In this work, numerical simulation calculations were performed to investigate the minimum ground temperature that occurs when extracting thermal energy in a horizontal ground heat exchanger system in the Central European climate. The influence of ground thermal conductivity, heat flux extracted from the ground, periodic interruptions in the operation of the heat exchanger, periodic supply of heat energy to the ground, relative humidity of the ambient air, evaporation rate coefficient, and convective heat transfer coefficient on the ground minimum temperature were investigated. Based on the simulation, it was found that the high value of ground thermal conductivity favorably affects the operation of the installation with a ground heat exchanger. Both the reduction of the maximum heat flux taken from the ground, as well as periodic interruptions in the operation of the exchanger effectively protects the ground against excessive cooling. Further, it was found that heat supply to the ground in summer only slightly raises its minimum temperature, as well as the decrease of the relative humidity of the ambient air and evaporation rate coefficient. The change of the convective heat transfer coefficient has no significant impact on the minimum annual ground temperature.

Research on Fresh and Hardened Sealing Slurries with the Addition of Magnesium Regarding Thermal Conductivity for Energy Piles and Borehole Heat Exchangers

Currently, renewable energy is increasingly important in the energy sector. One of the so-called renewable energy sources is geothermal energy. The most popular solution implemented by both small and large customers is the consumption of low-temperature geothermal energy using borehole heat exchanger (BHE) systems assisted by geothermal heat pumps. Such an installation can operate regardless of geological conditions, which makes it extremely universal. Borehole heat exchangers are the most important elements of this system, as their design determines the efficiency of the entire heating or heating-and-cooling system. Filling/sealing slurry is amongst the crucial structural elements. In borehole exchangers, reaching the highest possible thermal conductivity of the cement slurry endeavors to improve heat transfer between the rock mass and the heat carrier. The article presents a proposed design for such a sealing slurry. Powdered magnesium was used as an additive to the cement. The approximate cost of powdered magnesium is PLN 70–90 per kg (EUR 15–20/kg). Six different slurry formulations were tested. Magnesium flakes were used in designs A, B, C, and magnesium shavings in D, E and F. The samples differed in the powdered magnesium content BWOC (by weight of cement). The parameters of fresh and hardened sealing slurries were tested, focusing mainly on the thermal conductivity parameter. The highest thermal conductivity values were obtained in design C with the 45% addition of magnesium flakes BWOC.

Design, Development and Evaluation of Thermal Properties of Polysulphone–CNT/GNP Nanocomposites

Polysulphone (PSU) composites with carbon nanotubes (PSU-CNT) and graphene nanoplatelets (PSU-GNP) were developed through the solution casting process, using various weight load percentages of 1, 3, 5, and 10 wt% of CNT and GNP nanofillers. The microstructural and thermal properties of the PSU-based composites were compared. The microstructural characterisation of both composites (PSU-CNTs and PSU-GNPs) showed a strong matrix–filler interfacial interaction and uniform dispersion of CNTs and GNPs in the PSU matrix. The analysis demonstrated that both the thermal conductivity and effusivity improved with the increase in the weight percentage (wt%) of CNTs and GNPs because of the percolation effect. The polysulphone-based composite containing 10 wt% CNTs showed a remarkably high thermal conductivity value of 1.13 (W/m·K), which is 163% times higher than pure PSU. While the glass transition temperature (Tg) was shifted to a higher temperature, the thermal expansion was reduced in all the PSU-CNT and PSU-GNP composites. Interestingly, the CNTs allowed homogeneous distribution and a reasonably good interfacial network of interaction with the PSU matrix, leading to better microstructural characteristics and thermal properties than those of the PSU-GNP composites. The findings highlight the importance of controlling the nature, distribution, and content of fillers within the polymeric matrix.

Effect of nanostructured surface configuration on the interface properties and heat transfer of condensation process of argon inside nanochannels using molecular dynamics simulation

Present work studies the impact of surface structure on the heat transfer and liquid–vapor interface properties of condensation flow formed inside rough nanochannels compared to the smooth ones, using molecular dynamics simulation. Considering rectangular roughness configuration on the walls of nanochannel, simulations are performed under various saturated temperature conditions to obtain density, velocity, and temperature profiles. Furthermore, the effect of rough elements on the mechanism of liquid film formation and condensation rate as well as the thermal properties of two-phase flow has been investigated. It is observed that the amount of the oscillations in the vicinity of the walls at the end of the simulation time in rough nanochannels, is less than in smooth nanochannels. Results show that the effect of rough structures on the distribution of particles as well as velocity profile is greater at a lower temperature. Also, rough surfaces increase heat transfers at the beginning of the condensation process that leads to higher values of thermal conductivity in the rough nanochannel compared to the smooth nanochannel. Moreover, it is demonstrated that the thermal conductivity of argon condensation in rough nanochannel is approximately equivalent to the value of thermal conductivity in smooth nanochannel by adding one copper nanoparticle to the base fluid of argon.

Diborides of transition metals: Properties, application and production. review. Part 2. Chromium and zirconium diborides

The second part of the review considers properties, application and methods for producing chromium and zirconium diborides. These diborides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Chromium and zirconium diborides exhibit significant chemical resistance in aggressive environments. They have found application in modern technology because of these reasons. Chromium diboride is used as a sintering additive to improve the properties of ceramics based on boron carbide and titanium diboride. Zirconium diboride is a component of advanced ultra-high temperature ceramics (UHTC) ZrB2–SiC used in supersonic aircrafts and in gas turbine assemblies. Ceramics B4C–CrB2and B4C–ZrB2have high-quality performance characteristics, in particular, increased crack resistance. The properties of refractory compounds depend on the content of impurities and dispersion. Therefore, to solve a specific problem associated with the use of refractory compounds, it is important to choose the method of their preparation correctly, to determine the admissible content of impurities in the starting components. This leads to the presence of different methods for the borides synthesis. The main methods for their preparation are:a) synthesis from elements;b) borothermal reduction of oxides;c) carbothermal reduction (reduction of mixtures of metal oxides and boron with carbon;d) metallothermal reduction of metal oxides and boron mixtures;e) boron-carbide reduction. Plasma-chemical synthesis (deposition from the vapor-gas phase) is also used to obtain diboride nanopowders. Each of these methods is described.

Prediction of double transition metal (Cr1−xZrx)2AlC MAX phases as thermal barrier coatings: Insight from density functional theory

AbstractHerein, a high purity (Cr1−xZrx)2AlC solid solutions (with x = 0–1) have been investigated intensively using the first principles density functional theory calculations. The optimized structural parameters were founded to be in good agreement with the experimental results, and increased steadily with rising Zr in (Cr1−xZrx)2AlC solid solutions. The elastic constants Cij and enthalpy formation have been calculated and firmly confirmed that these solid solutions were mechanically and thermodynamically stable. Moreover, the mechanical properties show that the compounds possess brittle behavior with covalent bonding signified that they are relatively hard compared to other MAX phases. Further, the electronic band structures were studied and found a metallic behavior for all the investigated concentrations. Consequently, all the compositions of solid solutions were identified as promising materials for thermal barrier coating and high temperature structural materials due to the high value of melting temperatures and low minimum thermal conductivity.

Diborides of transition metals: Properties, application and production. Review. Part 1. Titanium and vanadium diborides

The properties, applications and methods for producing titanium and vanadium diborides are considered. These diborides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Titanium and vanadium diborides exhibit significant chemical resistance in aggressive environments. For these reasons, they have found application in modern technics. So, they are used as surfacing materials when applying wear-resistant coatings on steel products. It is also possible to use vanadium diboride as a catalyst in organic synthesis and the anode in renewable electrochemical current sources. Perspective are ceramics B4C – TiB2and B4C – VB2, which make it possible to obtain products based on boron carbide with high-quality performance characteristics, in particular, with increased crack resistance. Such composite ceramics are obtained by means of hot pressing, spark plasma sintering and pressureless sintering. The properties of refractory compounds depend on the content of impurities and dispersion. Therefore, to solve a specific problem associated with the use of refractory compounds, it is important to choose the method of their preparation correctly, to determine the admissible content of impurities in the starting components. This leads to the presence of different methods for the synthesis of borides. The main methods for their preparation are: synthesis from simple substances (metals and boron); borothermal reduction of oxides; carbothermal reduction (reduction of mixtures of metal oxides and boron with carbon; metallothermal reduction of mixtures of metal oxides and boron; carbide-boron reduction. Plasma-chemical synthesis (deposition from the vapor-gas phase) is also used to obtain diboride nanopowders. Each of these methods is characterized in the article.

A comparative study on the influences of whisker and conventional carbon nanotubes on the electrical and thermal conductivity of polyether ether ketone composites

AbstractIn this study, the electrical and thermal conductivity of polyether ether ketone (PEEK)/carbon nanotubes (CNTs) with different types, namely whisker CNTs (Wh‐CNTs) and conventional CNTs were compared. PEEK/CNTs composites were melt mixed in PEEK via two screw extrusion technology. Moreover, the effects of different methods for mixing of PEEK/Wh‐CNTs, namely, dry mixing, wet mixing and melt mixing, were compared. The electrical and thermal properties were analyzed. A high thermal conductivity value of about 0.741 W/(m K) could be obtained upon loading with 10 wt% Wh‐CNTs and the PEEK/Wh‐CNTs composites had low volume resistivity value of 10.96 Ω cm at 10 wt% loading via melt mixing. Thermal conductivity values of 1.066 W/(m K) (out‐of‐plane) and 1.50 W/(m K) (in‐plane) were achieved with 10 wt% loading by wet mixing. Experimental results of out‐of‐plane thermal conductivity were more consistent with the Nan model. PEEK/Wh‐CNTs composites prepared by wet mixing exhibited higher thermal conductivity than the composites mixed using the two other methods. As the content of Wh‐CNTs in wet mixed PEEK/Wh‐CNTs nanocomposites increased, electromagnetic interference (EMI) shielding effectiveness (SE) was improved. The PEEK/Wh‐CNTs composites were 0.6 mm thick and showed an EMI SE of 21.5 dB.

Thermophysical properties using ND/water nanofluids: An experimental study, ANFIS-based model and optimization

The research work was achieved to optimize the experimentally determined thermophysical properties of water based nanodiamond nanofluids using the Adaptive network-based fuzzy inference system model. The thermophysical properties were determined at various temperatures (20 °C to 60 °C) and various particle volume concentrations (0.2% to 1.0%). The stability of nanodiamond nanofluids was measured based on the dynamic light scattering method. Results indicated that the zeta potential of all the prepared nanodiamond/water nanofluids is above −30 mV. The thermophysical properties like thermal conductivity and viscosity augments are 22.86% and 79.16% at ϕ = 1.0 vol% compared to the water data at a temperature of 60 °C. Fuzzy logic is one of the artificial intelligence tools that have been used to analyze the optimized property value. The optimization process was applied in a single objective and a multi-objective procedure using a new and efficient optimizer, namely, the marine predators' algorithm. In a single objective, the lowest density and highest thermal conductivity values were found at 0.23% with 54.81o C and 1% with 60 °C, respectively. However, the viscosity and specific heat showed improvements only in the absence of the nanodiamond material and temperatures of 35.99 and 60 °C, respectively. In the multi-objective process, the optimizer confirmed that the thermophysical properties' optimal values could be obtained when no nanomaterial was added, and the best values were found at a temperature of 59.48 °C. This optimal point is found as close to the experimental data.

Investigation of an insulating waste conscious material for sustainable building application

One of the risks that threaten future generations is depletion of resources. Therefore, the exploitation of waste is one of the most successful ways to preserve resources and produce multi-use building materials. Calcium carbonate (CaCO3)/ Natural Rubber / Styrene-butadiene Rubber blend (NR/SBR) and (CaCO3)/foamed NR/SBR rubber blend were prepared by mastication -vulcanization technique. Specific gravity for all samples were determined while ranged between (0.42-1.4) for (CaCO3)/foamed rubber blend and between (1.003 – 2.9) for unfoamed rubber blend. Scanning electron microscope (SEM) for samples 150 phr of foamed and unfoamed rubber was done to compare the types of matrices. Thermal conductivity of the two groups were measured. The samples with concentrations till 150 phr CaCO3 foamed rubber lies in the range of thermal insulating materials while the samples of unfoamed rubber have higher thermal conductivity values. Water absorption test was conducted. The water absorption coefficient values of the foamed rubber are lower than that of unfoamed rubber. Mechanical measurements and swelling properties were investigated for all samples. The sample with 150 phr foamed rubber blend considered the optimum sample that achieve all desired thermal and moisture insulating properties.

Thermal and electrical properties of aluminum nitride/boron nitride filled polyamide 6 hybrid polymer composites

AbstractThe effects of boron nitride (BN) and aluminum nitride fillers on polyamide 6 (PA6) hybrid polymer composites were investigated. In particular, the thermal and electrical conductivity, thermal transition, thermal degradation, mechanical and morphological properties and chemical bonds characteristic of the materials with crystal structure of BN and aluminum nitride (AlN) filled PA6 prepared at different concentrations were characterized. Thermal conductivity of hybrid systems revealed a 1.6‐fold gain compared to neat PA6. The highest thermal conductivity value was obtained for the composite containing 50 vol% additives (1.040 W/m K). A slight improvement in electrical conductive properties of composites appears and the highest value was obtained for the 50 vol% filled composite with only an increase by 3%. The microstructure of these composites revealed a homogeneous dispersion of AlN and BN additives in PA6 matrix. For all composites, one visible melting peak around 220°C related to the α‐form crystals of PA6 was detected in correlation with the X‐ray diffraction results. An improved thermal stability was obtained for 10 vol% AlN/BN filled PA6 composite (from 405.41°C to 409.68°C). The tensile strength results of all composites were found to be approximately 22% lower than pure PA6.

Thermal vision of fracture behavior on acrylic composites

The aim of this work was to investigate thermal properties of acrylate-based composite combined with alumina-based particles. The composites were made of poly (methyl methacrylate) (PMMA) modified with dimethyl itaconate (DMI) as a matrix. As reinforcement were used alumina particles (Al2O3) and alumina doped with iron oxide (Al2O3-Fe) modified with 3-aminopropyl-trimethoxylane (AM) and flax oil fatty acid methyl esters (biodiesel - BD). According to the thermal conductivity measurements, the highest thermal conductivity values had the composite with alumina particles with the highest alpha phase content. With the addition of modified alumina particles to PMMA/DMI matrix mechanical properties were improved (tensile strength, modulus of elasticity and elongation at break). Composite with 3 wt. % Al2O3-Fe-AM particles had the most improved mechanical properties. It was noticed that this composite, compared to PMMA, had the lowest temperature in a crack zone (10.28%) and that is due to the better stiffness and highest thermal conductivity. The results indicated that the Sobel fractured surface area is related to the crack energy, which is reflected by the temperature of sample monitored by thermal vision.

Numerical Investigation of Free Convection in a Porous Corrugated Cavity Filled With Silver (Ag) Dispersed Nano-Fluid

Abstract The present work examines the convective heat transfer characteristics in a two-dimensional (2D) corrugated closed cavity embedded with porous media. The cavity is considered to be filled with silver dispersed water-based nano-fluid. The bottom wall is heated uniformly and non-uniformly in two different cases keeping both side corrugated walls isothermally cold with an adiabatic top wall. The various parameters are selected to perform numerical simulation in the range of solid-volume fraction, 0% ≤ ϕ ≤ 10%, 103 ≤ Ra ≤ 106, and 10−4 ≤ Da ≤ 10−2. The investigation shows that the heat transfer rate shows an increasing trend at high values of Ra and Da due to strong buoyancy forces in uniform and non-uniform heating. However, an increase in heat transfer in uniform heating is more compared to non-uniform heating. It is also observed that the nano-fluid has a great impact on the heat transfer characteristics due to its high value of thermal conductivity and irregular motion of the particles. As a result, the average Nusselt number (Nuavg) shows an increasing trend for increasing solid-volume fraction values.

High thermal conductivity of free-standing skeleton in graphene foam

Due to the high-porosity structure, the low thermal transport property of graphene foam (GF) is expected. However, the interconnected skeleton can still act as excellent thermal conductor branches if phonon scattering is not severely affected in the structure of graphene flakes. Such a property has not been validated experimentally due to the difficulty in sample manipulation and the fragility of the structure. In this work, we report the characterization results of thermal properties of the free-standing skeleton in GF. Three individual skeleton samples from one GF piece are prepared under the same condition. The thermal diffusivity of GF skeletons is characterized in the range of 3.26–3.48 × 10−4 m2/s, and the thermal conductivity is determined to be 520–555 W/(m K), which is two orders of magnitude larger than the value of bulk GF. These high thermal conductivity values originate from the intrinsic thermal property of graphene, while the contact interfaces, wrinkled structures, and defects induced in the synthesis process do not affect the phonon transport property significantly, which proves that the three-dimensional hierarchical graphene structure can still be implemented in energy-intensive applications.

Experimental study of solar adsorption refrigerator using a parabolic collector

Several alternative energy sources have been developed; one of them is solar energy to overcome the future energy crisis. Solar energy can be used as a source of heat energy, used very much, one of which is on the adsorption cycle refrigerator. The adsorption cycle refrigerator tested has three main components, namely the parabolic collector, condenser, and evaporator. In this research, the three main components are made of aluminum because they have a high thermal conductivity value and are resistant to corrosion due to water or refrigerant used. The pairs of adsorbent and refrigerant used were active powder carbon of 10 kg and methanol of 3 liters. The research purpose is to determine the maximum temperature at the collector, the minimum temperature in the cooled water, collector efficiency, and the COP value of the system. The experiment procedure is by heating the solar collector from 08.00 AM-05.00 PM (GMT+7), while the natural cooling process takes place from 05.00 PM-08.00 AM (GMT+7). The results obtained that the maximum temperature of the collector is 126.56°C. The minimum temperature of the water is -0.09°C, the maximum parabolic collector efficiency is 53.39%, and the maximum COP value is 0.000956.