Thermal Conductivity Of Powders Research Articles

Measurement of effective thermal conductivity of composite powders of 2D materials and metals for additive manufacturing

Laser Powder Bed Fusion (LPBF), a promising additive manufacturing technique for composites of 2D materials and metals, requires knowledge of thermophysical properties, such as the thermal conductivity of powder, for process optimization. In this study, we measured the effective thermal conductivity of the most representative Cu-graphene composite powder in the field of heat transfer applications. To measure thermal conductivity, we propose a differential scanning calorimetry (DSC) method to measure the thermal resistances of a powder bed under steady-state heat flow conditions. We observe that the thermal conductivity of a composite powder is 1,000 times lower than that of the bulk metal without 2D material addition; e.g., exhibiting ~ 0.30 W/mK in Cu-1wt.% graphene powder. Furthermore, we discuss powder size and morphology impact on thermal conductivity influencing the number of contact points and thermal contact resistance. Our findings contribute to understanding the thermal conductivity of composite powders in a powder bed and aid in optimizing LPBF processes.

Determination of Al2O3 Powder Thermal Conductivity Using DSC at NIS-Egypt

The mission of the thermal metrology laboratory is to maintain, disseminate, develop and realize the International Temperature Scale 1990 (ITS-90). One of the services that the laboratory has introduced is the routine calibration to all industrial sectors that covers almost all fields inside and outside Egypt. This paper explains how to use Differential Scanning Calorimeter (DSC) to determine the thermal conductivity of unknown powders. The paper is based on the ability to determine the thermal conductivity (k) of powders from the melting peak of the reference metal. This technique assists for the first time to establish a new activity at the National Institute for Standardization to test the thermal conductivity of powders, which had been requested by many clients. Experiments were carried out using alumina powder Al2O3 and metal reference beads of Sn and Zn which have diameter ranges from 1.9 mm up to 4.12 mm in an atmosphere of nitrogen during Sn and Zn melting. The results show good performance. The results showed confidence that will enable to do more studies at NIS to see if this technology can be applied to a wider spectrum of materials.

Numerical Simulation of Thermal Conductivity of Stainless Steel and Al-12Si Powders for Additive Manufacturing

Abstract A three-dimensional model of a partially melted powder bed with particles stochastically distributed in size and space coordinates has been developed. Numerical simulation of temperature distributions in stainless steel AISI 316 L and Al-12Si powders in vacuum, air, and argon has been performed to analyze unsteady heat transfer in a porous medium. The numerical model demonstrates a large effect of heat transfer through the gas phase in case of powders with low thermal conductivities like stainless steels. At the porosity level of 65% and above, the mechanism of heat transfer drastically changes and a linear dependence of thermal conductivity on porosity frequently used in literature becomes incorrect. The effects of the consolidation coefficient and size distribution on effective heat transfer in powders are discussed. The obtained dependencies of the effective thermal conductivity on porosity and the consolidation coefficient could be used in additive manufacturing applications.

Influence of packing density and fillers on thermal conductivity of polymer powders for additive manufacturing

Additive manufacturing of polymer powders is nowadays an industrial production technology. Complex thermal phenomena occur during processing, mainly related to the interaction dynamics among laser, powder, and heating system, and also to the subsequent cool-down phase from the melt to the parts. Thermal conductivity of the powder is a key property for material processing, since an inhomogeneous temperature field in the powder cake leads to uneven part properties and can strongly limit productivity because only a smaller portion of the build chamber can be used. Nevertheless, little is known about the relationship between thermal conductivity, packing density, and presence of fillers, which are used to enhance specific properties such as high temperature resistance or stiffness. The development and consequent validation of a device capable of measuring thermal conductivity as a function of powder packing density are then extremely important, providing an additional tool to characterize powders during the development process of new materials for PBF of polymers. The results showed a positive correlation between packing density and thermal conductivity for some commercially available materials, with an increase of the latter of about 10 to 40% with an increase of the packing density from 0 to 100%. Problems arose in trying to replicate the compaction state of the powder, since the same amount of taps led to a different packing density, but this is a known problem of measuring free-flowing powders such as the ones used for additive manufacturing. Regarding fillers, an increase of about 40 to 70% of thermal conductivity when inorganic fillers such as carbon fibers are added to the neat polymer was observed, and the expected behavior following the rule of mixture was only partially observed.

Powder thermal conductivity measurements in laser powder-bed fusion: an uncertainty study with sensitivity analysis

Laser flash testing and finite element (FE) heat transfer simulations have been together applied to measure the thermal conductivity of metallic powder contained in a solid specimen made by laser powder bed fusion (L-PBF) additive manufacturing. However, input parameters to the FE model potentially influence the accuracy of thermal conductivity evaluations. This study intends to investigate the effect of major uncertainties in the measurement of metallic powder thermal conductivity in L-PBF, including specimen dimensions, solid material properties, as well as the irradiation area and pyrometer detected area in laser flash testing, and to determine the sensitivity of various factors. A dummy-treated Taguchi method with different levels of the studied factors was utilized using the FE simulations and an inverse method. The results show that the dimension of the specimen’s internal powder-enclosed cavity and the solid material properties have dominant effects on the evaluation of powder thermal conductivity. In addition, predictions from the regression equations are verified and give a reasonable agreement with the simulated powder thermal conductivity values.

Studies on Thermal Conductivity and Thermal Stability of Cu-CNT Composite Powder Filled Epoxy Composites Using a Novel Approach for Sophisticated Thermal Management Applications

Studies on Thermal Conductivity and Thermal Stability of Cu-CNT Composite Powder Filled Epoxy Composites Using a Novel Approach for Sophisticated Thermal Management Applications

Analysis of Heat and Mass Transfer During Charging and Discharging in a Metal Hydride - Phase Change Material Reactor

This paper presents the heat and mass transfer study during charging and discharging processes in a metal hydride reactor (MHR) equipped with phase change material (PCM). The PCM numerical model is newly developed and incorporated into the metal hydride (MH) model, which was presented earlier [1]. It is based on the enthalpy-porosity formulation and considers source/sink terms in energy and momentum conservation equations. The study explores the effects of metal foam incorporated into PCM, and the variation of its porosity and pore density towards better heat transfer, and hydrogen charging and discharging rates. Detailed performance analysis of the MHR-PCM system with and without metal foam is performed, and the results show a significant improvement in performance with the PCM having metal foam embedded in it. The study on the effect of metal foam porosity reveals an interplay between effective PCM thermal conductivity and PCM amount on the hydrogen charging rate. The study clearly illustrates the limitation posed by the MH powder thermal conductivity on heat transfer and optimizes the metal foam morphology for improved hydrogen charging and discharging rates. The natural heat convection in the PCM is found to be negligible due to its high density. A brief study on the influence of hydrogen inlet and exit pressures, and the amount of PCM on charging and discharging rates is also performed. The study provides an in-depth insight into the formation of liquid and mass fraction fronts in the PCM and MH regions, respectively, under different operating conditions and PCM configurations.

Investigation of Thermal Properties of Epoxy Composites Filled with Alluminium Nitride (AlN)

Investigation of thermal properties of epoxy composites embedded with Aluminum Nitride (AlN) micro-fillers is estimated in the present thesis with help of the experimental and ANSYS results. Thermal conductivity of AlN powder filled epoxy composite is calculated by measuring temperature difference at two surfaces by using instruments thermal heater and thermal laser gun. The samples of composites prepared with AIN for three different sizes of AIN powder particles [AIN particles size 1)70µm 2)80µm 3)90µm]. For each size three samples are prepared with different concentration of AIN powder [AIN concentration by wt. 1)10 gm 2)20 gm 3)30 gm].The numerical results obtained by using the ANSYS software. The effective thermal conductivity values obtained from experimentally validate with ANSYS results. It is observed that The conductivity values measured experimentally are compared with the ANSYS results. These Epoxy composites with AlN and glass fiber have been fabricated and thermal conductivities of the samples are calculated by using experimentally obtained temperature values. Observations can be made that with At highest testing temp. 80oC for size 70µm of AIN particles we get thermal conductivity of epoxy composites for sample 20weight %(30gm AIN powder out of total weight of sample) has increased by 30 times than epoxy. Epoxy composites with 80µm size of AIN particles the sample with 20 % of AIN gives thermal conductivity at heater temp.80oC has increased by about 32 times than epoxy. For 90 µm size AIN particles samples at heater temp.80oC we found better thermal conductivity for sample with 20% AIN particles out of the nine samples the thermal conductivity of epoxy has increased by 34 times as compare to epoxy. A conclusion to draw from the measured values we got better thermal conductivity of epoxy composites with 20% of AIN with size of 90µm.Increase of thermal conductivity of epoxy composites with increase of size and percentage of AIN powder.

A Combined Experimental-Numerical Method to Evaluate Powder Thermal Properties in Laser Powder Bed Fusion.

Powder bed metal additive manufacturing (AM) utilizes a high-energy heat source scanning at the surface of a powder layer in a predefined area to be melted and solidified to fabricate parts layer by layer. It is known that powder bed metal AM is primarily a thermal process, and further, heat conduction is the dominant heat transfer mode in the process. Hence, understanding the powder bed thermal conductivity is crucial to process temperature predictions, because powder thermal conductivity could be substantially different from its solid counterpart. On the other hand, measuring the powder thermal conductivity is a challenging task. The objective of this study is to investigate the powder thermal conductivity using a method that combines a thermal diffusivity measurement technique and a numerical heat transfer model. In the experimental aspect, disk-shaped samples, with powder inside, made by a laser powder bed fusion (LPBF) system, are measured using a laser flash system to obtain the thermal diffusivity and the normalized temperature history during testing. In parallel, a finite element (FE) model is developed to simulate the transient heat transfer of the laser flash process. The numerical model was first validated using reference material testing. Then, the model is extended to incorporate powder enclosed in an LPBF sample with thermal properties to be determined using an inverse method to approximate the simulation results to the thermal data from the experiments. In order to include the powder particles' contribution in the measurement, an improved model geometry, which improves the contact condition between powder particles and the sample solid shell, has been tested. A multipoint optimization inverse heat transfer method is used to calculate the powder thermal conductivity. From this study, the thermal conductivity of a nickel alloy 625 powder in powder bed conditions is estimated to be 1.01 W/m K at 500°C. [DOI: 10.1115/1.4040877].

On the measurement of relative powder-bed compaction density in powder-bed additive manufacturing processes

Experimental studies in the literature have identified the powder-bed compaction density as an important parameter, governing the quality of additively manufactured parts. For example, in laser powder-bed fusion (LPBF), the powder-bed compaction density directly affects the effective powder thermal conductivity and consequently the temperature distribution in melt pool. In this study, this physical parameter in a LPBF build compartment is measured using a new methodology. A UV curable polymer is used to bind powder-bed particles at various locations on the powder-bed compartment when Hastelloy X was used. The samples are then scanned using a nano-computing tomography (CT) system at high resolution to obtain an estimation of the relative powder-bed compaction density. It is concluded that due to the interaction between the recoater and the variation in the powder volume accumulated ahead of the recoater across the build compartment, the relative powder-bed compaction density decreases along the recoater moving direction (from 66.4% to 52.4%.). This variation in the powder-bed compaction density affects the density and surface roughness of the final printed parts that is also investigated. Results show that the part density and surface quality decrease ~0.25% and ~20%, respectively, along the build bed in direction of the recoater motion.

Thermal conductivity of metal powders for powder bed additive manufacturing

The thermal conductivities of five metal powders for powder bed additive manufacturing (Inconel 718, 17-4 stainless steel, Inconel 625, Ti-6Al-4V, and 316L stainless steel) were measured using the transient hot wire method. These measurements were conducted with three infiltrating gases (argon, nitrogen, and helium) within a temperature range of 295–470 K and a gas pressure range of 1.4–101 kPa. The measurements of thermal conductivity indicate that the pressure and the composition of the gas have a significant influence on the effective thermal conductivity of the powder, but that the metal powder properties and temperature do not. Our measurements improve the accuracy upon which laser parameters can be optimized in order to improve thermal control of powder beds in selective laser melting processes, especially in overhanging and cellular geometries where heat dissipation by the powder is critical.

3ω slope comparative method for fluid and powder thermal conductivity measurements

By analyzing the relationship among the heat penetration depth, measurement frequency and detector characteristic parameters, a simple and practical [Formula: see text] slope comparative method has been proposed. The corresponding measurement system for measuring the thermal properties of fluids and powder materials was established and verified using several specimens with known thermophysical parameters, such as alcohol, distilled water, and air. Compared to the two-dimensional model, the data processing of the method is relatively simple and quick. Due to the elimination of errors introduced by the detector parameter measurement, the measurement accuracy of the method is higher than the conventional one-dimensional model. By using an appropriate frequency range, the new method is time saving and convenient for measuring the thermal conductivity of fluids and powders with low thermal conductivity. Based on the analysis, the effective thermal conductivity of nano-SiO2 powder is accurately determined.

Computation of Effective Thermal Conductivity of Powders for Selective Laser Sintering Simulations

In this work, a discrete element model (DEM) is developed and implemented in the open source flow solver MFiX to simulate the effective thermal conductivity of powder beds for selective laser sintering (SLS) applications, considering scenarios common in SLS such as thin beds, high temperatures, and degrees of powder consolidation. Random particle packing structures of spherical particles are generated and heat transfer between the particles is calculated. A particle–particle contact conduction model, a particle–fluid–particle conduction model, and a view factor radiation model using ray-tracing for calculation of view factors and assuming optically thick particles are used. A nonlinear solver is used to solve for the particle temperatures that drive the net heat transfer to zero for a steady state solution. The effective thermal conductivity is then calculated from the steady state temperature distribution. Results are compared against previously published experimental measurements for powder beds and good agreement is obtained. Results are developed for the impacts of very high temperatures, finite bed depth, consolidation, Young's modulus, emissivity, gas conductivity, and polydispersity on effective thermal conductivity. Emphasis is placed on uncertainty quantification in the predicted thermal conductivity resulting from uncertain inputs. This allows SLS practitioners to control the inputs to which the thermal response of the process is most sensitive.

Thermal conductivity of silica nanoparticle powder: Measurement and theoretical analysis

The hot-wire method was applied to experimentally determine the thermal conductivity (TC) of a silica nanoparticle powder. A fitting model was further employed to analyze the experimental results and to predict the TC over a wider porosity scale. Results show that the effective TC of the silica-nanoparticle powder can be less than that of free air because of the low TC of both the silica nanoparticles and the air confined in the pore spaces; the relative contribution of the nanoparticle TC, the confined air TC, and the radiation heat transfer coefficient to the effective TC will significantly affect at which porosity the extreme value of the effective TC occurs; the porosity obtained when the contribution to the effective TC of the confined air equals that of the nanoparticles is the most favorable for constructing thermal insulation materials.

Effective Thermal Conductivity of MOF-5 Powder under a Hydrogen Atmosphere

Effective thermal conductivity is an important thermophysical property in the design of metal-organic framework-5 (MOF-5)-based hydrogen storage tanks. A modified thermal conductivity model is built by coupling a theoretical model with the grand canonical Monte Carlo simulation (GCMC) to predict the effect of the H2 adsorption process on the effective thermal conductivity of a MOF-5 powder bed at pressures ranging from 0.01 MPa to 50 MPa and temperatures ranging from 273.15 K to 368.15 K. Results show that the mean pore diameter of the MOF-5 crystal decreases with an increase in pressure and increases with an increase in temperature. The thermal conductivity of the adsorbed H2 increases with an increased amount of H2 adsorption. The effective thermal conductivity of the MOF-5 crystal is significantly enhanced by the H2 adsorption at high pressure and low temperature. The effective thermal conductivity of the MOF-5 powder bed increases with an increase in pressure and remains nearly unchanged with an increase in temperature. The thermal conductivity of the MOF-5 powder bed increases linearly with the decreased porosity and increased thermal conductivity of the skeleton of the MOF-5 crystal. The variation in the effective thermal conductivities of the MOF-5 crystals and bed mainly results from the thermal conductivities of the gaseous and adsorption phases.

Thermal conductivity of powders used in continuous casting of steel

Measurements of the thermal conductivity (λ) and thermal diffusivity (α) have been carried out on samples of Na2O·SiO2 and four mould slags for temperatures between 295 and 1300 K. These measurements were carried out using the transient plane source (TPS) method (295–373 K) and the transient hot wire (THW) technique (295–1300 K) and the laser pulse (LP) or laser flash method in the case of Na2O·SiO2 Values of λ295 were very similar (1.07±0.03 W m−1 K−1) and all samples exhibited a maximum at the critical temperature (Tcrit = 1040±10 K) beyond which there was a sharp drop in λTHW with increasing temperature. Measurements were also carried out on semi-crystalline samples with increasing λ and fraction of crystalline phase. These samples also exhibited a sharp drop in the thermal conductivity for temperatures above Tcrit and λTcrit was found to be 1.6±0.05 W m−1 K−1. Thermal conductivity (λLP) values calculated from reported thermal diffusivity (αLP) values were in very good agreement with the measured λTHW values for temperatures between 295 K and Tcrit but λLP values did not decrease with increasing temperature above Tcrit (cf. λTHW). Consequently, it was found that λLP≈7λTHW for mould slags.

Thermal conductivity of powders used in continuous casting of steelPart 2 – Powders

Measurements of the thermal conductivity of casting powders are needed to determine the magnitude of the vertical heat flux in the powder bed of a continuous casting mould. The thermal conductivities and thermal diffusivities of four mould powders have been determined using the transient plane source (TPS) and the transient hot wire (THW) methods. The values reported in this investigation are in good agreement with the results of earlier studies which used the THW method. However, the results were significantly different from λeff values obtained in thermal insulation tests. This was attributed to the large contribution to λeff from gaseous conduction which arises from the large temperature gradient across the sample. It was found that the thermal conductivity (λpowd) of the powders had a mean value of 0.125±0.025 W m−1 K−1 for the four powders studied; this value is in good agreement with two other studies; it increased as the temperature increased with a mean temperature coefficient of (dλ/dT = 9×10−5 W m−1 K−2 for the range 295–1100 K and increased as the bulk density increased (λ295 = 0.010+1.69×10−4ρbulk W m−1 K−1).

Thermal conductivity of polyamide 12 powder for use in laser sintering

Purpose – The purpose of this paper is to acquire thermal conductivities of both fresh and preheated polyamide 12 powder under various conditions to provide a basis for effective and accurate control during the laser sintering (LS) process. Design/methodology/approach – A Hot Disk® TPS 500 thermal measurement system using a transient plane source (TPS) technology was employed for thermal conductivity measurements. Polyamide 12 powder was packed at different densities, and different carrier gases were used. Tests were also performed on fully dense laser sintered polyamide 12 to establish a baseline. Findings – Polyamide 12 powder thermal conductivity varies with packing density and temperature, which is approximately one-third bulk form thermal conductivity. Inter-particle bonding is the primary factor influencing polyamide 12 thermal conductivity. Research limitations/implications – Limited ranges of density were tested, and the carrier gas needed carefully control to prevent powder oxidation. Thermal properties obtained were not tested in the LS process. Originality/value – This experimental result could be used to enhance thermal control during the LS process.

Polyamide 11-MWNT nanocomposites: Thermal and electrical conductivity measurements

Multi-wall carbon nanotubes (MWNTs) with different loadings (0, 1, 3, 5, 7 and 9 wt%) were incorporated into polyamide 11 using a powder–powder mixing technique to form nanocomposites. The density and specific heat of each nanocomposite were measured. Thermal conductivities of these polymer nanocomposite powders were measured in a nitrogen atmosphere using a commercial transient plane-source device. Results indicate the thermal conductivity of preheated powder was larger than the fresh powder and similar to that at the melting temperature. The powder formulations were also pressed into films using a heat press and room temperature electrical conductivity measurements were performed. Substantial improvements in electrical conductivity were observed with increasing loading of MWNT. Microstructures of the nanocomposite specimens were examined using scanning electron microscopy.

Thermal Conductivity of Metal Powder and Consolidated Material Fabricated via Selective Laser Melting

Selective Laser Melting (SLM) is a direct fabrication of part through layer by layer powder deposition and successive laser beam irradiation based on Computer Aided Design (CAD) data. One of the important properties in SLM is thermal conductivity of metal powder. This is because the ability of metal powder to conduct heat will affect the consolidation process during SLM. In this paper, thermal conductivity of metal powders with different particle diameters and their mixture was analysed. Other than that, thermal conductivity of consolidated materials fabricated via SLM process was also studied. In order to measure the thermal conductivity of metal powder, a theoretically verified method which was previously developed by the authors was used. Determination of thermal conductivity of consolidated material was analysed using laser flash technique. It was found that the thermal conductivity of powder metal was influenced by bulk density and particle diameter of metal powder. In this study also, metal powders of different particle diameters were mixed with various volume ratios, and its effect was discussed. Thermal conductivity of the consolidated materials was also examined, and its relation to porosity was elaborated.