Microstructure and thermal conductivity of as-cast and as-extruded binary Mg–Mn alloys

This study investigated the microstructure and thermal conductivity of as-cast and as-extruded binary Mg–Mn alloys. The large grains and fine α-Mn particles contained in the as-cast alloys were observed. After extrusion, the average grain sizes significantly decreased, typical basal fibre texture was generated and high amount of nano particles precipitated from the α-Mg matrix. The thermal conductivity of both as-cast and as-extruded Mg–Mn alloys gradually decreased with the increase in Mn concentration. As-extruded alloys exhibited lower thermal conductivity than as-cast alloys except for the as-extruded alloys containing higher Mn concentrations (>1.2?wt-% Mn). Compared with binary as-extruded Mg–Al and Mg–Zn alloys reported in literatures, thermal conductivity of the as-extruded Mg–Mn alloys was higher when the solute content was greater than 0.5?at.-%.     

Thermal behaviour of gas atomised Al–20Mg–2Zr alloy powder

A novel ternary alloy with the composition of Al–20Mg–2Zr (wt-%) was prepared by close coupled gas atomisation. The thermal oxidation behaviour of the powder was examined by thermogravimetry–differential thermal analysis. The results showed that the oxidation proceeded in single step, and the violent exothermic reaction occurred after 900°C was almost complete. The activation energy of the oxidation was ～250?kJ?mol^??1, and the frequency factor was ～1.47?×?10¹¹?s^??1 and 3.36?×?10¹¹?s^??1 using the Kissinger and Ozawa method respectively. The special feature of the pulsed oxidation was explained by the melt dispersion oxidation mechanism. The excellent thermal reactivity exhibited by the Al–20Mg–2Zr powder suggested that this novel alloy could become one of the most promising materials in energetic applications.     

Characterisation and thermodynamic calculations of biodegradable Mg–2.2Zn–3.7Ce and Mg–Ca–2.2Zn–3.7Ce alloys

In the present study, the effect of Ca (0.5–6?wt-%) content on the microstructure, phase formation, and mechanical properties and in vitro degradation behaviour of Mg–2.2Zn–3.7Ce alloys were investigated. Microstructural analysis and thermodynamic calculations also showed that Mg–2.2Zn–3.7Ce alloy contain α-Mg, Mg₁₂Ce and CeMgZn₂, while after adding 0.5?wt-% Ca to Mg–2.2Zn–3.7Ce alloy, IM1 (Ca₃Mg_xZn_15?x) (4.6?≤?x?≤?12) phase was detected. Further addition of Ca to 6?wt-% resulted in forming Mg₂Ca besides α-Mg, Mg₁₂Ce and IM1 with the absence of CeMgZn₂. The tensile strength and elongation of the Mg–Ca–2.2Zn–3.7Ce alloys increase with increasing Ca content up to 1.5?wt-%, while further addition of Ca to 6?wt-% has a reversed effect. Similarly, the degradation rate of the alloys increased first with increasing Ca content and then decreased.     

3D Anisotropic Thermal Conductivity of Exfoliated Rhenium Disulfide

ReS₂ represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single‐crystalline ReS₂ in a distorted 1T phase is determined at room temperature for the in‐plane directions parallel and perpendicular to the Re‐chains, and the through‐plane direction using time‐domain thermoreflectance. ReS₂ is prepared in the form of flakes having thicknesses of 60–450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in‐plane thermal conductivity is higher along the Re‐chains, (70 ± 18) W m^?1 K^?1, as compared to transverse to the chains, (50 ± 13) W m^?1 K^?1. As expected from the weak interlayer bonding, the through‐plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m^?1 K^?1, resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in‐plane directions. The thermal conductivity and interface thermal conductance of ReS₂ are discussed relative to the other 2D materials.     

High thermal conductivity diamond coated graphite by microwave plasma chemical vapour deposition

Diamond film was grown on high thermal conductivity graphite substrate using microwave plasma chemical vapour deposition method. Nanodiamond particles were uniformly seeded on the substrate to generate high nucleation density by a spray gun. The continuous and high purity diamond film was obtained, and growth rate was up to 2.7 μm h^??1. The thickness, surface morphology, quality and composite phase of the film were analysed by SEM, Raman and X-ray diffraction. It was shown that graphite coated with diamond presented a higher thermal conductivity (520?W?m^??1 k^??1) than copper. Furthermore, this coated material with high thermal conductivity, good strength and non-conductive surface will make it possible to be widely used in thermal management field.     

Silica coating onto graphene for improving thermal conductivity and electrical insulation of graphene/polydimethylsiloxane nanocomposites

Graphene possess extremely high thermal conductivity, and they have been regarded as prominent candidates to be used in thermal management of electronic devices. However, addition of graphene inevitably causes dramatic decrease in electrical insulation, which is generally unacceptable for thermal interface materials(TIMs) in real electronic industry. Developing graphene-based nanocomposites with high thermal conductivity and satisfactory electrical insulation is still a challenging issue. In this study,we developed a novel hybrid nanocomposite by incorporating silica-coated graphene nanoplatelets(Silica@GNPs) with polydimethylsiloxane(PDMS) matrix. The obtained Silica@GNP/PDMS composites showed satisfactory electrical insulation(electrical resistivity of over 10~(13)Ωcm) and high thermal conductivity of 0.497 W m-1K-1, increasing by 155% compared with that of neat PDMS, even higher than that of GNP/PDMS composites. Such high thermal conductivity and satisfactory electrical insulation is mainly attributed to the insulating silica-coating, good compatibility between components, strong interfacial bonding, uniform dispersion, and high-efficiency heat transport pathways. There is great potential for the Silica@GNP/PDMS composites to be used as high-performance TIMs in electronic industry.     

Microstructure and mechanical properties of asextruded Mg–xAl–5Sn–0·3Mn alloys (x = 1, 3, 6 and 9)

Properties of Mg–xAl–5Sn–0·3Mn alloys (x?=?1, 3, 6 and 9) prepared by hot extrusion are reported. The orientation relationship between Mg₂Sn precipitate and Mg matrix in Mg–9Al–5Sn–0·3Mn alloy was determined. The yield strength of the as extruded alloys initially decreased with increasing Al content, then increased for Al contents >6 wt-%. These changes are interpreted in terms of the effect of texture, grain size and second phase on the yield strength of the alloys.     

High‐Performance Thermally Conductive Phase Change Composites by Large‐Size Oriented Graphite Sheets for Scalable Thermal Energy Harvesting

Efficient thermal energy harvesting using phase‐change materials (PCMs) has great potential for cost‐effective thermal management and energy storage applications. However, the low thermal conductivity of PCMs (K_PCM) is a long‐standing bottleneck for high‐power‐density energy harvesting. Although PCM‐based nanocomposites with an enhanced thermal conductivity can address this issue, achieving a higher K (>10 W m^?1 K^?1) at filler loadings below 50 wt% remains challenging. A strategy for synthesizing highly thermally conductive phase‐change composites (PCCs) by compression‐induced construction of large aligned graphite sheets inside PCCs is demonstrated. The millimeter‐sized graphite sheet consists of lateral van‐der‐Waals‐bonded and oriented graphite nanoplatelets at the micro/nanoscale, which together with a thin PCM layer between the sheets synergistically enhance K_PCM in the range of 4.4–35.0 W m^?1 K^?1 at graphite loadings below 40.0 wt%. The resulting PCCs also demonstrate homogeneity, no leakage, and superior phase change behavior, which can be easily engineered into devices for efficient thermal energy harvesting by coordinating the sheet orientation with the thermal transport direction. This method offers a promising route to high‐power‐density and low‐cost applications of PCMs in large‐scale thermal energy storage, thermal management of electronics, etc.     

Characteristics of ultrahigh electrical conductivity for Cu–Sn alloys

Abstract

The resistivity and mechanical properties of Cu–Sn alloys with different compositions were explored by casting, normalising, cold work and subsequent annealing treatment. Results indicated that the Cu–Sn alloy had the characteristics of ultrahigh electrical conductivity, when the Sn content was ～0·5 wt-%. Note that the resistivity of the as cast and annealed Cu–0·5 wt-%Sn alloys is 1·55 and 1·26 μΩ cm respectively.     

Bulk thermal conductivity studies of Sn/SnO coated and filled multiwalled carbon nanotubes for thermal interface material

Multiwalled carbon nanotubes (MWCNTs) were successfully coated and filled with Sn using a simple method. Bulk thermal conductivity of Sn/SnO filled and coated multiwalled carbon nanotubes (MWCNTs) of ～1 mm thickness pellet by laser method reveals surge in hybrid carbon nanotubes in comparison to pristine nanotubes. The thermal diffusivity and thermal conductivity of hybrid nanotubes are increased to 4.41 mm²/sec, 5.39 Wm^?1K^?1 of as compared to 0.36 mm²/sec and 0.28 Wm^?1K^?1 of the pristine nanotubes. The enhancement in thermal conductivity is attributed to the presence of Sn coating on surface and inside the carbon nanotubes and the formation of compact structures by reducing the air gaps between nanotubes because of their joining during compression and sintering.     

Influence of alloying elements on thermal conductivity and high temperature strength of copper based alloys

Abstract

The effects of alloying elements and annealing temperature on thermal conductivity and softening behaviour of Cu – 0·1Ag – xP–yMg and Cu – xSn – yTe alloys (all compositions are in wt-%) have been investigated. The Cu – 0·1Ag – xP–yMg alloys, in spite of greater amounts of P and Mg, had a higher electrical conductivity and a higher softening temperature than those of a Cu – 0·1Ag – 0·031P alloy. A Cu – 0·032Sn – 0·023Te alloy had the same levels of electrical conductivity and softening temperature as those of Cu – 0·040Sn. The conductivity and softening temperature of the Cu – 0·032Sn – 0·023Te alloy are comparable with those of the Cu – 0·1Ag – 0·013P alloy currently used for continuous casting mould materials.     

Effects of minor additions of Ce and Y on as cast microstructure of Mg–3Sn–2Ca magnesium alloy

Abstract

The effects of minor additions of Ce and Y on the as cast microstructure of Mg–3Sn–2Ca (wt-%) magnesium alloy are investigated and compared. Results indicate that adding minor Ce or Y to Mg–3Sn–2Ca alloy does not cause formation of any new phases in the alloy. The as cast Mg–3Sn–2Ca alloy with addition of 0·5 wt-%Ce or Y is still composed of α-Mg, CaMgSn and Mg₂Ca phases. However, after adding 0·5 wt-%Ce or Y to Mg–3Sn–2Ca alloy, not only the formation of CaMgSn phase in the alloy is suppressed but also the CaMgSn phases in the alloy are effectively refined. In addition, adding 0·5 wt-%Ce to Mg–3Sn–2Ca alloy exhibits higher refinement efficiency to the CaMgSn phase in the alloy than adding 0·5 wt-%Y. Further investigations need to be considered in order to understand the difference of minor Ce and Y with regard to the refinement of CaMgSn phase in the Mg–3Sn–2Ca alloy.     

Quantum-Confinement-Enhanced Thermoelectric Properties in Modulation-Doped GaAs–AlGaAs Core–Shell Nanowires

Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core–shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core–shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65–85 µV K⁻¹ are observed for the lowest sub-bands, resulting in equivalent thermopower of S²σ ≈ 60 µW m⁻¹ K⁻² and S²G ≈ 0.06 pW K⁻² within a single sub-band. Remarkably, these core–shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m⁻¹ K⁻¹, about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.     

Thermal conductivity and mechanical properties of Sm-containing Mg-Zn-Zr alloys

This study investigated the effect of Sm additions on the microstructure, thermal conductivity, and mechanical properties of Mg-Zn-Zr alloy. The results indicate that the addition of Sm led to the formation of a rare-earth phase at the grain boundaries, and the grain size was significantly refined in the extruded state. The thermal conductivity of Mg alloy increased with the increase in Sm content because of the formation of a rare-earth phase that helps to dissolve the Zn atoms in the α-Mg matrix. Moreover, the as-extruded Mg alloy exhibited a higher thermal conductivity (up to124?W?(m?K)^?1) than its as-cast counterparts. The Sm-containing as-extruded Mg alloy showed excellent yield strength of up to 254?MPa, and also a good plastic deformation ability.     

Thermal conductivity improvement of copper–carbon fiber composite by addition of an insulator: calcium hydroxide

The effects of adding calcium hydroxide (Ca(OH)₂) to a copper–CF (30 %) composite (Cu–CF(30 %)) were studied. After sintering at 700 °C, precipitates of calcium oxide (CaO) were included in the copper matrix. When less than 10 % of Ca(OH)₂ was added, the thermal conductivity was similar to or higher than the reference composite Cu–CF(30 %). A thermal conductivity of 322 W m^?1 K^?1 was measured for the Cu–Ca(OH)₂(3 %)–CF(30 %) composite. The effects of heat treatment (400, 600, and 1000 °C during 24 h) on the composite Cu–Ca(OH)₂(3 %)–CF(30 %) were studied. At the lower annealing temperature, CaO inside the matrix migrated to the interface of the copper matrix and the CF. At 1000 °C, the formation of the interphase calcium carbide (CaC₂) at the interface of the copper and CFs was highlighted by TEM observations. Carbide formation at the interface led to a decrease in both thermal conductivity (around 270 W m^?1 K^?1) and the coefficient of thermal expansion (CTE (10.1 × 10^?6 K^?1)).     

Thermal properties of Mg–Li and Mg–Li–Al alloys

Abstract

The present work is a study of the thermal properties of Mg–xLi–y Al with x= 4, 8 and 12 wt-% and y= 0, 3 and 5 wt-% as a function of temperature in the range 20–375°C. The thermal diffusivity and coefficient of thermal expansion (CTE) have been measured and the thermal conductivity calculated. The thermal diffusivity of all alloys decreases with an increasing content of lithium. The CTE of the single phase alloys Mg–4Li and Mg–12Li has a linear character, and the CTE of Mg–12Li is higher than that of Mg–4Li. The influence of thermal stresses in the two phase alloy Mg–8Li is perceptible in terms of temperature dependence of the CTE. In Mg–4Li–3Al and Mg–4Li–5Al, an influence of the solution of AlLi phase on all the studied thermal properties has been found.     

Copper effects in mechanical,thermal and electrical properties of rapidly solidified eutectic Sn–Ag alloy

The Sn–3.5 wt%Ag alloy considered as a good alternative to Pb–Sn alloys. This study aims to investigate the effects of Cu or Sb additions by 3 or 5 wt% to melt-spun Sn–3.5%Ag alloy. Ternary melt-spun Sn–Ag–Cu and Sn–Ag–Sb alloys investigated using X-ray diffractions (XRD), Scanning electron microscope (SEM), Dynamic resonance technique (DRT), Instron machine, Vickers hardness tester and Differential scanning calorimetry (DSC). The results revealed that the microstructures of the β-Sn phase, Ag₃Sn and Cu₃Sn intermetallic compounds (IMCs) in the solder matrices were refined due to the effect of Cu additions and melt-spun process. Moreover, increasing Cu content promotes Ag₃Sn intermetallic compound (IMC) formation. Consequently, the addition of “3 wt%” of Cu reduced the creep rate ? from (3.79?×?10^?3) to (1.65?×?10^?3) and delayed the fracture point. The tensile results showed an improvement in Young’s modulus by 47% (30.3 GPa), ultimate tensile strength (UST) by 11.6% (23.9 MPa), and in toughness by 20.5% (952.32 J/m³) compared to the eutectic Sn–Ag alloy. Vickers hardness has improved by 3.3% (136.71 MPa) and thermal activation energy by 54% (90.40 KJ/mol) when compared with that of eutectic Sn–Ag alloy. Those improvements are related to the lack of lattice strain from 7.56?×?10^?4 without “3 wt%” of Cu to 5.26?×?10^?4 with “3 wt%” of Cu. Its melting temperature (T_m) increased by 3 °C due to Ag₃Sn IMC increased and Cu₃Sn formation, but the pasty rang (mushy zone) decreased by 4 °C with “3 wt%” of Cu. The small lattice strains resulted with “3 wt%” of Cu made the electrical resistivity of this alloy more stable at elevated temperatures. The mechanical, thermal and electrical improvements of Sn_93.5–Ag_3.5–Cu₃ alloy provide good physical performance for soldering process and electronic assembly.     

Ultrahigh Power Factor and Ultralow Thermal Conductivity at Room Temperature in PbSe/SnSe Superlattice: Role of Quantum-Well Effect

Reduced dimension is one of the effective strategies to modulate thermoelectric properties. In this work, n-type PbSe/SnSe superlattices with quantum-well (QW) structure are fabricated by pulsed laser deposition. Here, it is demonstrated that the PbSe/SnSe multiple QW (MQW) shows a high power factor of ≈25.7 µW cm^?1 K^?2 at 300 K, four times larger than that of PbSe single layers. In addition, thermal conductivity falls below 0.32 ± 0.06 W m^?1 K^?1 due to the phonon scattering at interface when the PbSe well thickness is confined within the scale of phonon mean free path (1.8 nm). Featured with ultrahigh power factor and ultralow thermal conductivity, ZT at room temperature is significantly increased from 0.14 for PbSe single layer to 1.6 for PbSe/SnSe MQW.     

Electrostatic interaction-based self-assembly of paraffin@graphene microcapsules with remarkable thermal conductivity for thermal energy storage

Graphene encapsulating paraffin (paraffin@graphene) microcapsules were fabricated by electrostatic interaction-based self-assembly. An aqueous dispersion of graphene sheets charged with cation, were mixed with a water-based emulsion containning negatively charged paraffin droplet spheres to form self-assembled microcapsules. The morphology of the microcapsules was characterized by scanning electron microscope (SEM). Results show that the microcapsules with a well-defined spherical structure were prepared successfully. Differential scanning calorimeter (DSC) results indicate that the phase change latent heat are all above 200?J g^?1. With a graphene mass fraction of 8?wt%, the thermal conductivity of the fabricated composites can reach 1.73?W m^?1 K^?1. Attributing to the interlocking of graphene with each other, the microcapsules enable lock the paraffin in the shell thus successfully avoiding its leakage during phase change process. The prepared phase change microcapsules are expected to apply in energy storage field.     

Microstructure and mechanical properties of Mg–6Li–xAl–0.8Sn alloys

As-cast and as-extruded Mg–6Li–xAl–0.8Sn (x?=?0, 1, 3 and 5?wt-%) alloys were prepared. The microstructure and mechanical properties were investigated and discussed. The experimental results show that the Mg–6Li–0.8Sn alloy is composed of three phases: α-Mg, Mg₂Sn and Li₂MgSn. With the addition of Al, the test alloys display typical α-Mg?+?β-Li duplex structures. The new Mg₁₇Al₁₂ and LiMgAl₂ phases were found in the Mg–6Li–1Al–0.8Sn alloy. The lamellar-type AlLi phase was formed whereas the Mg₁₇Al₁₂ phase disappeared in Mg–6Li–3Al–0.8Sn alloy. The LiMgAl₂ phase vanished in the Mg–6Li–5Al–0.8Sn alloy. The mechanical properties of as-extruded alloys were remarkably improved. The as-extruded Mg–6Li–3Al–0.8Sn alloy exhibited the best mechanical properties, with a yield strength, tensile strength and elongation of 209.8?MPa, 242.6?MPa and 15.5%, respectively.