Conductivity Coefficient Research Articles

A novel solar power generation hybrid system comprising evacuated U-tube solar collector and thermally regenerative thermocapacitive cycle

While solar energy offers a promising solution for sustainable energy, there’s a continuous drive to explore alternative methods of solar power generation. This paper introduces a novel solar power generation hybrid system that merges an angle-independent evacuated U-tube solar collector (EUSC) with a thermally regenerating thermocapacitive cycle (TRTC). Under AM 1.5 conditions, the hybrid system is anticipated to achieve a power density of 44.33 W m−2, accompanied by an efficiency of 4.43 %, underscoring its great prospect for practical applications. To unlock the performance potential, considerable parametric studies are conducted to find out diverse strategies to boost the hybrid system’s efficacy, encompassing the elevation of EUSC’s inlet temperature, reduction of external environmental temperature, moderation of fluid velocity within EUSC, and enhancement of thermal conductivity in the TRTC. Through meticulous optimization of thermal conductivity between TRTC and the environment, the hybrid system’s theoretical efficiency can attain a maximum of 9.38 %. A gradient-based local sensitivity analysis pinpoints that the EUSC’s internal fluid flow velocity is the most sensitive while the thermal conductivity coefficient between the TRTC and EUSC is the least sensitive. These results uncover key information critical for efficiently designing and operating the hybrid system.

Enhanced n-Type Thermoelectric Properties and Structure Evolution of Carbonized Metal-Coordination Polydopamine.

Carbonized polydopamine (cPDA) exhibits a nitrogenous graphite-like structure with n-type semiconductor property. However, the low electrical conductivity and Seebeck coefficient of cPDA cannot meet the needs of flexible thermoelectric devices. In this study, a series of metal ions were coordinated with cPDA to enhance n-type thermoelectric properties. At 300 K, all metal-coordination cPDA (metal-cPDA) samples obtain lower thermal conductivity compared to cPDA. Mn-cPDA exhibits the greatest Seebeck coefficient of -25.94 μV K-1, which is almost six times higher than cPDA. Fe-cPDA shows the best electrical conductivity of 2.45 × 105 S m-1. An optimal power factor (PF) value of 11.93 μW m-1 K-2 is achieved in Ca-cPDA with the enhanced electrical conductivity of 9.5 × 104 S m-1 and Seebeck coefficient of -11.24 μV K-1. Using Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), we revealed the structural characterization of metal-cPDA. Our results indictate that the different metal ions (Mn2+, Zn2+, Mg2+, Al3+, Ca2+, and Fe3+) exert varying influences on the growth of graphite-like structure within metal-cPDA, which lead to the evolution of electrical conductivity. We observe that the carrier density and carrier mobility depend on both the degree of graphitization and the metal-ion coordination, which work together on electrical conductivity and Seebeck coefficient. These findings and understanding of the thermoelectric properties of PDA-based materials will help to realize high-performance n-type thermoelectric materials for flexible electronic device applications.

Improved thermoelectric performance in polypyrrole/Cu2SnS3 composites

Polypyrrole (PPy) is a promising thermoelectric organic material due to low thermal conductivity, higher electrical conductivity, easy preparation and stability. However, its thermoelectric performance is limited by the intrinsic low Seebeck coefficient. In this paper, PPy/xCu2SnS3 (x = 0–5 wt%) composites with enhanced thermoelectric properties were prepared through chemical oxidation method using iron chloride (FeCl3) as oxidant and methyl orange (MO) as soft template. The obtained composites were investigated by X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and Raman spectroscopy. The Cu2SnS3 increased the carrier density and electrical conductivity, meanwhile, thanks to the energy filtering effect and phonon scattering at the organic-inorganic interface of PPy and Cu2SnS3, the Seebeck coefficient increased and thermal conductivity decreased for PPy/ xCu2SnS3 (x > 0) composites. The highest electrical conductivity (74.571 S/cm) and Seebeck coefficient (11.165 μV/K) at 300 K were obtained from PPy/xCu2SnS3 (x = 1 wt%), resulting in a maximum figure of merit (ZT) of 1.646×10−3, which is 8.5 times higher than that of pure PPy.

Effect of hBN and SDS added vegetable based cutting fluid application on the performance of turning Ti6Al4V alloys: A Comparative analysis with Taguchi and ANN approaches

In this study, during the turning process of Ti6Al4V alloy, which is difficult to machine, nanofluid, whose base fluid is vegetable oil (sunflower oil), was applied to the cutting zone with the minimum quantity lubrication (MQL) technique. Nanofluids were prepared by adding hexagonal boron nitride (hBN) nanoparticles (65–75 nm) with different concentration ratios (0.5 and 1 %) and surfactant (Sodium dodecyl sulfate, SDS) into vegetable oil. Thermal conductivity coefficients and dynamic viscosities of the prepared nanofluids and pure sunflower oil were measured at four different temperatures. Additionally, the stability of the prepared nanofluids was observed for 15 months. In turning experiments, four different cutting speeds (50, 75, 100 and 125 m/min), four different feed rates (0.05, 0.10, 0.15 and 0.20 mm/rev) and four different cooling conditions (dry, PureMQL, 0.5 % NanoMQL and 1 % NanoMQL) were used. The effect of the change in these parameters on surface roughness (Ra), temperature in the cutting zone (T) and flank wear on the tools (Vb) was investigated experimentally and analytically. As a result of the research, the experimental results were evaluated using variance analysis, signal/noise (S/N) analysis and artificial neural networks (ANN) methods. As a result of the measurements and observations, it was determined that the nanofluid with 0.5 % concentration had the best stability. In the main experiments, the MQL condition using this nanofluid (0.5 % NanoMQL) exhibited the best machining performance. In the wear experiments, the lowest tool wear was detected under the 1 % NanoMQL condition. According to the ANOVA and S/N analysis results showed that Ra was most affected by the feed rate (95 %), T was most affected by the cooling condition (70 %), and Vb was most affected by the cutting speed (45 %). The prediction performances of ANN and Taguchi approaches were examined and the prediction success of these approaches was more than 98.5 % and 80.8 %, respectively. Especially for Vb values that have very close data, more accurate predictions were made with the ANN approach compared to Taguchi.

MACHINE LEARNING METHODS AS APPLIED TO MODELLING THERMAL CONDUCTIVITY OF EPOXY-BASED COMPOSITES WITH DIFFERENT FILLERS FOR AIRCRAFT

The thermal conductivity coefficient of epoxy composites for aircraft, which are reinforced with glass fiber and filled with aerosil, γ-aminopropylaerosil, aluminum oxide, chromium oxide, respectively, was simulated. To this end, various machine learning methods were used, in particular, neural networks and boosted trees. The results obtained were found to be in good agreement with the experimental data. In particular, the correlation coefficient in the test sample was 0.99%. The prediction error of neural networks in the test samples was 0.5; 0.3; 0.2%, while that of boosted trees was 1.5; 0.9%.

Microstructure, durability and surface free energy of lightweight aggregate modification of sanitary ceramic wastes and sewage sludge

The research on the durability and physical properties of lightweight aggregate (LWA) with addition of sanitary ceramic wastes and sewage sludge was presented in the paper. The following characteristics of LWA were defined: contact angle (CA), absorptivity, roughness, surface free energy (SFE) before and after the UV radiation durability test, thermal conductivity coefficient λ, compressive strength, freezing-thawing, salt resistance and others. Open porosity was examined using computed tomography, it reached 20.987 % for the ceramic waste aggregate, and 7.023 % for the reference aggregate. The aggregate with the largest amount of ceramic waste (20 %) and sewage sludge (10 %) has the highest average roughness (Ra), which is 14 % higher than the Ra of the reference aggregate. The contact angle decreased by almost 4 times and samples had higher absorptivity (19.994 %) when 10 % sewage sludge and 20 % sanitary ceramic were added to the aggregate. The sanitary ceramic waste application may enhance the poor durability characteristics of lightweight aggregate with/without sewage sludge. Replacing natural loess with sanitary ceramic waste material brings benefits both in terms of respect for natural resources and also improves the properties of lightweight aggregates.

A Comparative Analysis of Low and High SiC Volume Fraction Additively Manufactured SiC/Ti6Al4V(ELI) Composites Based on the Best Process Parameters of Laser Power, Scanning Speed and Hatch Distance.

Silicon carbide (SiC) exhibits intriguing thermo-physical properties such as higher heat capacity and conductivity, as well as a lower density than Ti6Al4V(ELI). These properties make SiC a good candidate for the reinforcement of Ti6Al4V(ELI) with respect to its use as a heat shield in aero turbines to increase their efficiency. The traditional materials used in aircraft structures were required to have a combination of good mechanical properties such as strength, stiffness, and hardness and low weight, as well as low thermo-physical properties such as coefficient of thermal expansion (CTE) and thermal conductivity. The alloy Ti6Al4V(ELI) has a density of 4.45 g/cm3, which is lower than that of structural steel (7.4 g/cm3) and higher than that of aluminium (2.5 g/cm3). Lower density benefits light weighting. Aluminium is the lightest of the traditional materials used but has relatively low strength. The CTE of SiC of 4.6 × 10-6/K is lower than that of Ti6Al4V(ELI) of 8.6 × 10-6/K, while the density of SiC of 3.21 g/cm3 is lower than that of Ti6Al4V(ELI) of 4.45 g/cm3. Therefore, from the theory of composites, SiC/Ti6Al4V(ELI) composites are expected to have lower densities and CTEs than those of Ti6Al4V(ELI), thus providing for lightweighting and less thermal related buckling or separation at their joints with carbon/epoxy resin panels. The specific strength, stiffness, and Knoop hardness of SiC of 75-490 kNm/kg, 132 MNm/kg, and 600-3800 GPa, respectively, are generally larger than those of Ti6Al4V(ELI) of 211 KNm/kg, 24 MNm/kg, and 880 GPa, respectively. Therefore, investigating reinforcement of Ti6Al4V(ELI) with SiC particles is worthwhile as it will lead to the formation of composites that are stronger, stiffer, harder, and lighter, with lower values of CTE. For additive manufacturing, this requires initial studies to optimise the process parameters of laser power and scanning speed for single tracks. To print single tracks in the present work, different laser powers ranging from 100 W to 350 W and scanning speeds ranging from 0.3 m/s to 2.7 m/s were used for different SiC volume fraction values of values. To print single layers, different values of hatch distance were used together with the best values of laser power and scanning speed determined elsewhere by the authors for different volume fractions of SiC. Through optical microscopy, the built tracks and their cross sections were examined. By using laser power and scanning speeds of 200 W and 1.2 m/s, and 150 W and 0.8 m/s, respectively, the best tracks at 5% and 10% volume fractions were obtained, whereas the best tracks at 25% volume fraction were achieved using a laser power of 200 W and a scanning speed of 0.5 m/s. Furthermore, the results showed that the maximum SiC volume percentage of 30% resulted in limited or no penetration. Therefore, it is concluded from the study that parts with improved mechanical properties can be produced at SiC volume fractions ranging from 5% to 25%, while parts produced at the high volume fraction of 30% would have unacceptable mechanical qualities for the final part.

The curing kinetics and properties of self-healing and thermally conductive polymeric composites based on ethylene vinyl acetate copolymer filled with nano alumina

We have investigated a self-healing thermally conductive Ethylene-vinyl acetate (EVA) hot melt adhesive (NAEDS, which denotes the addition of nano-alumina and disulfide bonded EVA hot melt adhesives) that can be used to effectively solve adhesive layer separation problem in photovoltaic (PV) modules and the difficulty of recycling. More specifically, we used diallyl disulfide as a cross-linking agent for EVA and alumina nanofillers as a reinforcing phase. The nano alumina filler gave the EVA better thermal conductivity. We used FTIR, SEM, and DSC to characterize and analyze NAEDS properties. The disulfide bonds in NAEDS underwent a rearrangement reaction under ultraviolet light, resulting in the self-healing of the resin (self-healing efficiency of 78.8%). The added nano alumina not only reduced the resin curing activation energy, improving its curing performance but also enhanced its thermal conductivity, achieving a 2.61 W/(m*k) thermal conductivity coefficient. The overall NAEDS performance is improved.

Rice Husk-Based Insulators: Manufacturing Process and Thermal Potential Assessment.

The development of bio-insultation materials has attracted increasing attention in building energy-saving fields. In tropical and hot-humid climates, building envelope insulation is important for an energy efficient and comfortable indoor environment. In this study, several experiments were carried out on a bio-insulation material, which was prepared by using rice husk as a raw material. Square rice husk-based insultation panels were developed, considering the ASTM C-177 dimensions, to perform thermal conductivity coefficient tests. The thermal conductivity coefficient obtained was 0.073 W/(m K), which is in the range of conventional thermal insulators. In a second phase of this study, two experimental enclosures (chambers) were constructed, one with rice husk-based insulation panels and the second one without this insulation. The measures of the temperatures and thermal flows through the chambers were obtained with an electronic module based on the ARDUINO platform. This module consisted of three DS18B20 temperature sensors and four Peltier plates. Daily temperature and heat flux data were collected for the two chambers during the dry season in Panama, specifically between April and May. In the experimental chamber that did not have rice husk panel insulation on the roof, a flow of up to 28.18 W/m2 was observed, while in the chamber that did have rice husk panels, the presence of a flow toward the interior was rarely observed. The rice husk-based insulation panels showed comparable performance with conventional insulators, as a sustainable solution that takes advantage of a local resource to improve thermal comfort and the reduction of the environmental impact.

Study of the physical properties of the Li2CuO2 compound

In this paper, we investigate the electronic, thermodynamic, Elastic and optical properties of the compound Li2CuO2 by using the density functional theory (DFT) under the Hubbard correction term U. Such approach allows us to gain an in-depth apprehension of the band structure, density of states, as well as the specific optical properties of compound Li2CuO2. Our results revealed that the studied material Li2CuO2 displays energy band gap values of 2.008 eV for the up spin structure and 1.582 eV for the down spin structure. In particular, the indirect bandgap is situated at the Γ point of the valence band maximum (VBM) and the M point of the conduction band minimum (CBM) points of the Brillouin zone. Moreover, we carried out a more precise analysis of the optical properties of the Li2CuO2 compound, including the dielectric function, absorption coefficient, conductivity, and extinction coefficient. These parameters give essential informations for evaluating the optical behavior of the Li2CuO2 in different practical applications. In particular, we have also investigated the thermodynamic properties of the studied compound, such as the heat capacity and the Debye temperature. This contributes to a better understanding of the compound's thermal behavior. We believe that our findings are of crucial importance in evaluating the material's stability and reactivity under various thermal circumstances.

Spherical partial differential equation with non‐constant coefficients for modeling of nonlinear unsteady heat conduction in functionally graded materials

AbstractThe objective of this research paper is to propose an exact solution for resolving the transient conduction in a 2D sphere. The coefficients of governing equation are varied according to the material properties. The thermo‐physical properties are regarded as functions that follow a power‐law relationship concerning the radial direction. Both radial and angular thermal conductivity coefficients change with radius. Thermal boundary conditions are considered in a general state, which can cover different thermal conditions, including Dirichlet, Neumann, and Convection surface conditions. Laplace transform and Meromorphic function methods are used in the solution approach to the current unsteady problem. Two unsteady case studies with complex boundary conditions have been considered to show the credibility of the current solution. The results of both case studies have been successfully validated. The results confirm the high capability of the present solution in solving unsteady thermal problems of functionally graded materials in spherical coordinates.

Thermally conductive polymer materials with high dielectric characteristics based on oligodimethylsiloxane with terminal hydroxyl groups

The results of studying a polyfunctional heat-resistant composition based on oligodimethylsiloxane with terminal hydroxyl groups filled with boron nitride are presented. The influence of boron nitride on the physical-mechanical, thermophysical and electrical properties of the sealing composition is shown. The dependence of thermal conductivity coefficient, electrical resistivity and other characteristics on the content of the modifier has been established.

The Effect of Radiant Heat on Polystyrene Thermal Insulation Materials

As a result of the increasingly stringent energy requirements of recent years, the issue of façade thermal insulation is gaining more and more prominence. Due to these requirements, layer thickness of insulation increases, or the use of materials with a better coefficient of thermal conductivity is preferred. These requirements are met by polystyrene insulation materials. Therefore, it is extremely important to know as much as possible the behaviour of polystyrene thermal insulation under the influence of fire and radiant heat. Its regulation is not coordinated in Europe, and the effect of radiant heat is not addressed by any country’s standard. In this paper, the behaviour of polystyrene thermal insulations, which are densely used in Hungary is examined, under the influence of radiant heat.

Order-N calculations for thermoelectric power factor based on linear response theory

We present an order-N quantum transport calculation methodology to evaluate thermoelectric transport coefficients, such as electric conductivity and Seebeck coefficient. Different from a conventional method using the electric conductivity spectrum, it obtains the coefficients directly from the correlation function between heat and electric current based on linear response theory. As an example, we apply the methodology to a two-dimensional square-lattice model with static disorder and confirm that the calculated results are consistent with those obtained by the conventional method. The proposed methodology provides an effective approach to evaluate the thermoelectric performance of micron-scale materials based on quantum mechanics from an atomistic viewpoint.

Effects of ion doping on the dielectric and electro-optical properties of chiral nematic liquid crystal

This study presents experimental findings pertaining to the use of the cationic surfactant cetyltrimethylammonium bromide (CTAB) as a dopant dispersed in cholesteric liquid crystal. The impact of the additive on the electrical and dielectric properties was investigated by dielectric spectroscopy. Compared with that in the pure counterpart, the dielectric relaxation frequency increased due to the added ions in the cholesteric liquid crystals impregnated with 0.01–0.5-wt% CTAB. Furthermore, by analyzing the dielectric spectra in a specific frequency range, the DC conductivity, ion density, and diffusion coefficient were calculated and compared. By fitting the experimental data into the Arrhenius equation, the activation energy was obtained as well. Our experimental results illustrated that all these physical properties were strongly dependent on the CTAB concentration and the temperature. This work also involved the electro-optical measurement in combination with the observation of optical texture. The ion addition provoked electrohydrodynamic flows in a surfactant-dispersed mixture, generating a homogeneous and transparent uniform lying helix state between two low-transmission optical states—the focal conic and dynamic scattering states. The operating frequency range of the dynamic scattering state was significantly broadened by the ionic dopant.

Multi-criteria assessment of groundwater potential of shallow aquifers in the vicinity of Osun state university, Ikire campus and environs, southwestern Nigeria

In order to investigate the groundwater potentials of shallow water-bearing layers within the vicinity of the Osun State University Campus at Ikire, fifteen vertical electrical sounding (VES) resistivity data were acquired at random. The groundwater potential of the shallow water-bearing layers was inferred from the resistivity data based on three (3) existing criteria namely aquifer resistivity, transmissivity and reflective coefficient. At each VES station, the maximum electrode spreading employed for the resistivity data acquisition was between 200 m and 240 m, with the interpretation of the data carried out using partial curve-matching and computer iterative software. Geoelectric parameters obtained were used to compute the longitudinal conductance, transverse resistance, hydraulic conductivity, transmissivity and reflective coefficient using empirical relationships. The computed parameters were compared with existing classification tables to deduce the groundwater propensity in the shallow water-bearing layers of the study area. Four to five geoelectric layers corresponding to the HQ, HA and HKA geoelectric curve types were observed having lateritic materials as topsoil, sandy clay/clayey sand, weathered/fractured basement and fresh basement. In most cases, the water-bearing layer is mostly of clayey material with high porosity and low permeability and resistivity values that varies from 14.6 Ωm in VES 1 to 65.6 Ωm in VES 6. A few consist of sandy clay also with low resistivity values 76.9 Ωm in VES 7 to 107.1 Ωm in VES 4. The transmitivity values vary from 0.239 m2/day in VES 2 to 2.522 m2/day in VES 5. This same behavior was observed with reflective coefficient values which range from 0.690 in VES 6 to 0.994 in VES 1. The conclusions from the classification tables correlate in most of the VES stations. This study shows that the groundwater potential of the shallow water-bearing layers in the study area is generally poor within 0 to 80 m depth due to the low transmissivity of the clay materials at the shallow depths. Effective groundwater development in the study area must therefore integrate geophysical methods and remote sensing techniques to investigate depths beyond 100 m.

Strong anisotropic transport properties of quasi-one-dimensional ZrTe3 nanoribbons

Transition metal trichalcogenides (TMTCs) with quasi-one-dimensional (quasi-1D) structures have attracted extensive interest in diverse fields because of their unique electronic properties. Herein, we report strong in-plane anisotropic transport properties of ZrTe3 nanoribbons, which originate from their quasi-1D structures. ZrTe3 nanoribbons were mechanically exfoliated from bulk single crystals, and transport properties, including electrical conductivity (σ) and Seebeck coefficient (S), were measured in the longitudinal and transverse directions of the rectangular nanoribbons. σ was higher in the transverse direction than that in the longitudinal direction. S measurement revealed further clear anisotropic characteristics, and positive and negative S values were obtained in the longitudinal and transverse directions, respectively. Furthermore, a significant change in the transport properties was observed with a post treatment, and the potential of ZrTe3 for transport property modulation was confirmed. We believe that our findings will contribute to the understanding of anisotropic transport in quasi-1D ZrTe3 and expansion of the application fields of quasi-1D ZrTe3.

Divacancy and resonance level enables high thermoelectric performance in n-type SnSe polycrystals

N-type polycrystalline SnSe is considered as a highly promising candidates for thermoelectric applications due to facile processing, machinability, and scalability. However, existing efforts do not enable a peak ZT value exceeding 2.0 in n-type polycrystalline SnSe. Here, we realized a significant ZT enhancement by leveraging the synergistic effects of divacancy defect and introducing resonance level into the conduction band. The resonance level and increased density of states resulting from tungsten boost the Seebeck coefficient. The combination of the enhanced electrical conductivity (achieved by increasing carrier concentration through WCl6 doping and Se vacancies) and large Seebeck coefficient lead to a high power factor. Microstructural analyses reveal that the co-existence of divacancy defects (Se vacancies and Sn vacancies) and endotaxial W- and Cl-rich nanoprecipitates scatter phonons effectively, resulting in ultralow lattice conductivity. Ultimately, a record-high peak ZT of 2.2 at 773 K is achieved in n-type SnSe0.92 + 0.03WCl6.

The Impact of Volume Fraction and Nanoparticles in the Flow of Nanofluids along the Boundary Layer Over a Flat Plate

Introduction: The forced convective, steady, viscous, nonlinear boundary layer flow of a two-dimensional problem over a flat plate with heat transfer nanofluids. The flow is laminar. Two different types of nanoparticles, namely, Titanium dioxide () and Silver are used to prepare nanofluids with base fluid is chosen as water. Nanofluids has a large number of applications in the area of biomedical, cooling and tribological applications. Objectives: To study about the effect different nanoparticles and their volume fraction in the flow of nanofluids along the boundary layer over a flat plate. Analyse the changes in the viscosity, density, specific heat capacity, thermal conductivity, Prandtl number, skin friction coefficient and Nusselt number. Methods: The flow of the fluid that are governed by the PDEs that are nonlinear are transformed into the ODEs that are nonlinear by employing similarity transformations, which then solved by the use of MATLAB bvp5c solution procedure. Results: Both thermal conductivity and heat transfer coefficient increases with the increase in the volume fraction of the corresponding nanoparticle. Also, Silver-water nanofluid has the high thermal conductivity and heat transfer rate when compared with Titanium dioxide-water nanofluid. Conclusions: Silver water nanofluid surpasses titanium dioxide water nanofluid in terms of thermal conductivity and heat transfer rate. Moreover, the physical attributes like viscosity, density, and thermal conductivity of nanofluids escalate proportionally with the volume fraction of nanoparticles. Conversely, specific heat capacity and Prandtl number exhibit an inverse relationship with the volume fraction of nanoparticles, decreasing as it increases.

A Simulation Study on Utilizing in Airbus380 fuel Tanks for Thrust and Fuel Efficiency

This paper explores the potential benefits of using nanofluids within the fuel system of an Airbus A380 to enhance engine thrust and reduce fuel consumption. By analyzing the Nusselt and Reynolds numbers, the study aims to understand the heat transfer and fluid dynamics that could lead to more efficient aircraft operation. The integration of nanotechnology in aviation fuel systems represents a promising frontier for improving aircraft performance. the nanoscale as a fluid whose particles are suspended between 1-100 nanometers and permanently in the particle. Adding nanoscale increases heat transfer. In this research used silicon carbide nanofluid (sic nanofluid) with a percentage of (1,2,3,4%) the base fluid (Jet A1) as well as thermal properties including Nusselt number and thermal conductivity coefficient and... It's been studied.