Thermal conductivity of carbon nanotube reinforced aluminum composites: A multi-scale study using object oriented finite element method

In this study, object oriented finite element method (OOF) has been utilized to compute the thermal conductivity of plasma sprayed Al-12 wt.% Si containing 10 wt.% multiwall carbon nanotubes (CNTs). The computations have been made at micro- and macro-length scales which highlight the effect of CNT dispersion on thermal conductivity. Experimentally measured values at 50 °C indicate that CNT addition reduced the thermal conductivity of Al–Si matrix from 73 W m⁻¹ K⁻¹ to 25.4 W m⁻¹ K⁻¹ which is attributed to the presence of CNT clusters. OOF calculations at micro-length scale predicted an 81% increase in the conductivity of Al–Si matrix due to presence of well dispersed CNTs inside the matrix. At larger lengths scale, the decrease in the overall conductivity is related to the extremely low conductivity of CNT clusters. Thermal conductivity of CNT clusters could be up to three orders of magnitude lower than individual CNTs. OOF computed values match well with experimental results. OOF compute thermal conductivity of Al–CNT composite is also compared with theoretical two-phase models for CNT-composites at different length scales.     

Structural and electrochemical properties of nanostructured nickel silicides by reduction and silicification of high-surface-area nickel oxide

Nanostructured nickel silicides have been prepared by reduction and silicification of high-surface-area nickel oxide (145 m² g^?1) produced via precipitation. The prepared materials were characterized by nitrogen adsorption, X-ray diffraction, thermal analysis, FT-IR spectroscopy, scanning electron microscopy, transmission electron microscopy, magnetic and electrochemical measurements. The nickel silicide formation involves the following sequence: NiO (cubic) → Ni (cubic) → Ni₂Si (orthorhombic) → NiSi (orthorhombic) → NiSi₂ (cubic), with particles growing from 13.7 to 21.3 nm. The nickel silicides are ferromagnetic at room temperature, and their saturation magnetization values change drastically with the increase of Si content. Nickel silicides have remarkably low electrical resistivity and noble metal-like properties because of a constriction of the Ni d band and an increase of the electronic density of states. The results suggest that such silicides are promising candidates as inexpensive yet functional materials for applications in electrochemistry as well as catalysis.     

Growing polystyrene chains from the surface of graphene layers via RAFT polymerization and the influence on their thermal properties

The polystyrene (PS) macromolecular chains were grafted on the surface of graphene layers by reversible addition-fragmentation chain transfer (RAFT) polymerization. In this procedure, a RAFT agent, 4-Cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid, was used to functionalize the thermal reduced graphene oxide (TRGO) to obtain the precursor (TRGO-RAFT). It can be calculated that the grafting density of PS/graphene (PRG) composites was about 0.18 chains per 100 carbons. Successful in-plain attachment of RAFT agent to TRGO and PS chain to TRGO-RAFT was shown an influence on the thermal property of the PRG composites. The thermal conductivity (λ) improved from 0.150 W m⁻¹ K⁻¹ of neat PS to 0.250 W m⁻¹ K⁻¹ of PRG composites with 10 wt% graphene sheets loading. The thermal property of PRG composites increased due to the homogeneous dispersion and ordered arrangement of graphene sheets in PS matrix and the formation of PRG composites.     

Nanocomposite protective coatings based on Ti–N–Cr/Ni–Cr–B–Si–Fe,their structure and properties

The first results of manufacturing and investigations of a new type of nanocomposite protective coatings are presented. They were manufactured using a combination of two technologies: plasma-detonation coating deposition with the help of plasma jets and thin coating vacuum-arc deposition. We investigated structure, morphology, physical and mechanical properties of the coatings of 80–90 μm thickness, as well as defined the hardness, elastic Young modulus and their corrosion resistance in different media. Grain dimensions of the nanocomposite coatings on Ti–N–Cr base varied from 2.8 to 4 nm. The following phases and compounds formed as a result of plasma interaction with the thick coating surface were found in the coatings: Ti–N–Cr (200), (220), γ-Ni₃–Fe, a hexagonal Cr₂–Ti, Fe₃–Ni, (Fe, Ni)N and the following Ti–Ni compounds: Ti₂Ni, Ni₃Ti, Ni₄Ti, etc. We also found that the nanocomposite coating microhardness increased to H = 31.6 ± 1.1 GPa. The Young elastic modulus was determined to be E = 319 ± 27 GPa – it was derived from the loading–unloading curves. The protective coating demonstrated the increased corrosion resistance in acidic and alkaline media in comparison with that of the stainless steel substrate.     

Initial phase hot corrosion mechanism of gas tunnel type plasma sprayed thermal barrier coatings

The hot corrosion resistance of the top layer in TBC is one of the main constructive factors which determines the lifetime of the coatings under critical operating environments. In the present study, 8 wt% yttria stabilized zirconia (8YSZ), lanthanum zirconate (La₂Zr₂O₇) and equal weight percentage of its composite (50%8YSZ + 50% La₂Zr₂O₇) coatings were prepared by using gas tunnel type plasma spray torch at optimum spraying conditions. The hot corrosion performances of the above thermal barrier coatings were examined against 40 wt%V₂O₅–60 wt%Na₂SO₄ corrosive ash at 1173 K for 5 h in open air atmosphere. After hot-corrosion testing, the coating surface was studied using a scanning electron microscope to observe the microstructure and X-ray diffraction techniques were used to identify the phase compositions. The results showed that LaVO₄ and YVO₄ are the main hot corrosion products along with the ZrO₂ phase transformation from tetragonal to monoclinic phases in La₂Zr₂O₇ and 8YSZ coatings respectively. The microstructure and phase formation mechanism of the hot corrosion products varied with each coating and among these, composition of 50%8YSZ + 50%La₂Zr₂O₇ coating exhibited least degradation against V₂O₅–Na₂SO₄ corrosive environment compared to the other coatings.     

Microhardness and corrosion properties of hypoeutectic Al–7Si alloy processed by high-pressure torsion

High-pressure torsion (HPT) was used to produce hypoeutectic Al–7Si alloy samples having a range of microstructures to investigate the effect of the grain refinement on its corrosion behavior in 3.5 wt.% NaCl solution for the first time. Optical microscopy measurements reveal that with the HPT processing increased from 1/4 to 10 revolutions under an applied pressure of 6.0 GPa, brittle coarse silicon particles and intermetallic phases were effectively broken into ultrafine-grained particles and redistributed homogeneously into the Al-rich matrix. Open-circuit potential and polarization curves results exhibit that corrosion resistance of the Al–7Si alloy in NaCl solution was significantly enhanced upon high torsion strains, with corrosion rate reduced from 7.41 μm y⁻¹ for the as-received sample to 1.68 μm y⁻¹ for the 10-turn processed sample. Electrochemical impedance spectroscopy analysis combined with characterization of the corroded samples using scanning electron microscopy and energy dispersive X-ray spectroscopy indicates that the enhancement in corrosion performance of the Al–7Si alloy is due to the breakage of coarse silicon particles and intermetallic phases, the microstructure homogeneity and the increased HPT-induced active sites. It is demonstrated that microstructure refinement through HPT processing can significantly improve both microhardness and corrosion properties of the Al–7Si alloy.     

Preparation of coatings with high adhesion strength and high corrosion resistance on sintered Nd–Fe–B magnets through electroless plating

Electroless Ni–P-based coatings have been deposited on sintered Nd–Fe–B magnets through applying ultrasonic irradiation and adjusting the [Cu²⁺]/[Ni²⁺] ratio in the solution. The effects of the ultrasonic power on the adhesion to the magnet substrate and the [Cu²⁺]/[Ni²⁺] ratio on the corrosion resistance of the coatings were investigated. It was found that the adhesion of the coating to the substrate could be greatly improved through applying ultrasonic irradiation. Maximum adhesion strength reached 56 MPa at 150 W. The results also showed that the addition of Cu²⁺ could improve the corrosion resistance of Ni–P-based coatings. When the [Cu²⁺]/[Ni²⁺] ratio was 0.02, the coating could be as long as 512 h free of corrosion in the neutral salt spray. The compact amorphous structure was responsible for the improved corrosion resistance of the coating.     

Studies of carbon behavior in tungsten under carbon and hydrogen mixed irradiation

The erosion of W coatings with the different content of C was investigated in Ar and Ar + H₂ plasmas. The enhanced erosion was registered in Ar + H₂ plasma for coatings containing 40 at.% of C and more, which might be owing to chemical erosion of carbon. The erosion kinetics was analyzed together with the studies of coatings microstructure, phase composition and Vicker hardness properties.The C distribution profiles in W were investigated in dependence on the redeposition rate of sputtered C and simultaneous 0.3 keV Ar⁺ ion irradiation. It is shown that C penetration depth depends on the coverage of W by C during ion irradiation. Deep C penetration was registered when W was partially covered by redeposited carbon.The computer simulation was used to extrapolate obtained results to experimental investigations performed by Ueda Y, Fukumoto M, Sawamura I, Sakizono D, Shimada T, Nishikawa M. [Fusion Eng Des 2006;81:233–9]. It is revealed that the ballistic relocation of implanted C atoms is the dominant process which explains the deep penetration of C and formation of WC compound in the near-surface region of W.     

High-rate pulsed reactive magnetron sputtering of oxide nanocomposite coatings

The article reports on the oxide nanocomposite coatings reactively sputtered by a pulsed dual magnetron and is divided into two parts. The first part briefly describes main problems in the reactive sputtering of oxides, i.e. low deposition rate a_D and arcing at the target surface and then focuses on the discharge of the dual magnetron. The ways how a_D can be increased and arcing eliminated are shown. The second part is devoted to transparent oxide coatings. Two types of oxide coatings are described in detail: (1) Si–Zr–O coatings containing ≤5 at.% of Zr and (2) Zr–Al–O coatings with Zr/Al > 1. It is shown that (a) Si–Zr–O coatings exhibit high thermal stability up to 1500 °C, almost 100% optical transparency and can be deposited with very high a_D ≈ 800 nm/min from a molten magnetron target and (b) Zr–Al–O coatings with relatively high hardness H ≈ 18–19 GPa, low effective Young’s modulus E¹ satisfying the ratio H/E¹ > 0.1 are highly elastic (the elastic recovery W_e > 70%) and exhibit an enhanced resistance to cracking. The last finding is of key importance for development of new hard coatings with enhanced toughness.     

One-step preparation of dense TiC1 − xNx–Ni3Ti cermet by combustion synthesis

TiC_1 − xN_x-based cermets are attractive for high-speed cutting operations, and are usually produced by powder sintering at high temperatures and for a long time. In this paper, dense TiC_1 − xN_x–Ni₃Ti cermet is directly prepared in one step by combustion synthesis in 2 MPa N₂ atmosphere, from the reactant of (Ti + C + Ni) with a molar ratio of Ti:C:Ni = 1:0.7:0.4. The as-prepared TiC_1 − xN_x–Ni₃Ti cermet shows a homogeneous microstructure, in which equiaxed TiC_1 − xN_x grains of a few microns are bounded by Ni₃Ti grain-boundary phase, and a Vickers hardness of 12.1 ± 0.3 GPa. It is proposed that the deficiency of C in the starting reactant plays a key role in simultaneous densification during combustion synthesis. Compared with the conventional powder sintering approach, combustion synthesis may provide a one-step, ultrafast, and furnace-free way to prepare TiC_1 − xN_x-based cermets with reduced time and energy consumption.     

Improving the hot corrosion resistance of plasma sprayed ceria–yttria stabilized zirconia thermal barrier coatings by laser surface treatment

Ceria–yttria stabilized zirconia (CYSZ) thermal barrier coatings (TBCs) were deposited by air plasma spraying on NiCoCrAlY-coated Inconel 738LC substrates. After that, the surface of plasma sprayed CYSZ TBCs were glazed using a pulsed Nd:YAG laser. The effects of laser glazing on hot corrosion resistance of the coatings were evaluated in presence of 45 wt%Na₂SO₄ + 55 wt%V₂O₅ corrosive molten salt at 1000 °C. The results revealed that the hot corrosion resistance of plasma sprayed CYSZ TBCs were enhanced more than twofold by laser surface glazing due to reducing specific reactive area of the dense glazed surface layer and consequently, decreasing the reaction between molten salt and zirconia stabilizers.     

Influence of thermal treatment on the structure,humidity sensitivity,electrical and magnetic properties of barium–tungsten ferrite

The purpose of this study was to analyze the methods and effects of introducing tungsten ions in a hexagonal barium ferrite structure by partial substitute of barium with tungsten. For this investigation we prepared a tungsten substitution on the barium hexaferrite Ba_0.5W_0.5Fe₁₂O₁₉ using sol–gel self-combustion technology. The powder was treated for 30 min, without atmosphere precautions, at 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C. Scanning electron microscopy has revealed the crystallite size and shape, and the X-ray diffraction was provided information related to the phase compositions. The investigation was focused on the variation of permittivity and electrical resistivity, in relation to the treatment temperature, frequency and humidity. We have also investigated the influence of thermal treatment on to Curie temperature and frequency characteristics of the relative permeability. Because barium–tungsten ferrite shows a porous structure, the measurements are strongly influenced by humidity conditions, and in this respect we analyzed the variations with humidity of permittivity and electrical resistivity.     

Microstructure and thermal expansion behavior of spray-deposited Al–50Si

The evolution of microstructure and coefficient of thermal expansion (CTE) of the Al–50Si (wt.%) alloy manufactured by spray deposition followed by hot isostatic pressing (HIP) are systematically investigated. The results indicate that the microstructure of the deposited alloy is composed of primary Si with average size of 12.5 ± 0.1 μm and α-Al. The CTE of the deposited alloy is higher than the corresponding alloy produced by casting due to the high solid solubility of Al in Si. After HIP, the CTE is lower than the parent as-deposited alloy owing to the high solid solubility of Si in Al. The residual thermal stress results in a higher CTE during the second heating as a result of the CTE mismatch between the Al matrix and the primary Si particles. Furthermore, the measured CTE value is in good agreement with the Turner model after complete densification by HIP at 843 K.     

Analysis of tensile strength and microstructure of Ni–SiC composites prepared by electroforming

Ni–SiC metal matrix composites with two kinds of SiC content were prepared by electroforming in a nickel sulphamate bath. Tensile strength and microstructure of the composites before and after heat treatment were investigated. The maximum of tensile strength was obtained after heat treatment at 300 °C × 24 h. The values were 641 N/mm² and 701 N/mm² respectively. The complete reaction between nickel and SiC particles can produce shrinkage pores in the interface. The volume of shrinkage pores was equal to 8% of the volume of SiC particles in the composites. The interfacial reaction products were composed of Ni₃Si and a little amount of Ni₃₁Si₁₂ after heat treatment at 600 °C × 24 h. The fracture evolution went though microcracks initiation, growth and coalescence. Cracking of the matrix, debonding of Ni–SiC interfaces and cracking of particles were three types of cracking modes for Ni–SiC composites.     

Binder-free graphene/carbon nanotube/silicon hybrid grid as freestanding anode for high capacity lithium ion batteries

Light-weight graphene/Si (G/Si) hybrid binder-free electrode is deemed a high energy density anode contender for lithium ion batteries (LIBs). However, paper-like G/Si electrodes tend to show an increased migration distance for Li⁺ through the fast interlayer channel with the increment of electrode size, in addition to an intrinsically slow diffusion kinetics; thereby encumbering their commercial realisation in high energy density and long life LIBs. To address these problems, herein, sandwich-structured graphene/carbon nanotube/silicon (G/CNT/Si, Si: 56 wt.%, ∼500 nm) hybrid grid is designed, cognizant of its uniform and shorter Li⁺ migration distance. Cyclic voltammograms indicate G/CNT/Si paper and grid anode to exhibit good electrochemical activity at both low and high temperatures. Noteworthy is that the Li⁺ diffusion coefficient ratio between G/CNT/Si grid and paper anodes are 1.82, 1.64, 1.43, 1.36 and 1.53 at a temperature of −5, 10, 25, 40 and 55 °C, respectively. The initial coulombic efficiencies of both paper and grid anode are as high as ∼82%. After 60 cycles at 420 mA g⁻¹, the charge capacity of G/CNT/Si grid is retained at 808 mA h g⁻¹, which by far surpasses that of paper anode (i.e., 490 mA h g⁻¹). The attained lithium ion storage performance at both high and low temperatures, underpins the G/CNT/Si sandwiched grid as effective to realise the practical deployment of paper-like graphene electrodes for high energy density and long life LIBs.     

Influence of heat treatment on microstructure and adhesion of Al2O3 fiber-reinforced electroless Ni–P coating on Al–Si casting alloy

Influence of heat treatment regime on microstructure, phase composition and adhesion of Al₂O₃ fiber-reinforced Ni–P electroless coating on an Al–10Si–0.3 Mg casting alloy is investigated in this work. The pre-treated substrate was plated using a bath containing nickel hypophosphite, nickel lactate and lactic acid. Al₂O₃ fibers pretreated with demineralised water were placed into the plating bath. Resulting Ni–P–Al₂O₃ coating thickness was about 12 μm. The coated samples were heat treated at 400–550 °C/1–8 h. LM, SEM, EDS and XRD were used to investigate phase transformations. Adhesion of coating was estimated using scratch test with an initial load of 8.80 N. It is found that annealing at high temperatures (450 °C and above) leads to the formation of hard intermetallic products (namely Al₃Ni and Al₃Ni₂ phases) at the substrate–coating interface. However, as determined by the light microscopy and by the scratch test, these phases reduce the coating adhesion (compared to coatings treated by the optimal annealing regime 400 °C/1 h). The analysis of scratch tracks proves that fiber reinforcement significantly reduces the coating scaling. However, due to the formed intermetallic sub-layers, partial coating delamination may occur on the samples annealed at 450 °C and above.     

Effect of metalloid silicon addition on densification,microstructure and thermal–physical properties of Al/diamond composites consolidated by spark plasma sintering

Aluminum matrix composites reinforced with diamond particles were consolidated by spark plasma sintering. Metalloid silicon was added (Al–Si/diamond composites) to investigate the effect. Silicon addition promotes the formation of molten metal during the sintering to facilitate the densification and enhance the interfacial bonding. Meanwhile, the alloying metal matrix precipitates the eutectic-Si on the diamond surfaces acting as the transitional part to protect the improved interface during the cooling stage. The improved interface and precipitating eutectic-Si phase are mutually responsible for the optimized properties of the composites. In this study, for the Al–Si/diamond composite with 55 vol.% diamonds of 75 μm diameter, the thermal conductivity increased from 200 to 412 Wm⁻¹ K⁻¹, and the coefficient of thermal expansion (CTE) decreased from 8.9 to 7.3 × 10⁻⁶ K⁻¹, compared to the Al/diamond composites. Accordingly, the residual plastic strain was 0.10 × 10⁻³ during the first cycle and rapidly became negligible during the second. Additionally, the measured CTE of the Al–Si/diamond composites was more conform to the Schapery’s model.     

Surface microstructure and cell biocompatibility of silicon-substituted hydroxyapatite coating on titanium substrate prepared by a biomimetic process

Silicon-substituted hydroxyapatite (Si-HA) coatings with 0.14 to 1.14 at.% Si on pure titanium were prepared by a biomimetic process. The microstructure characterization and the cell compatibility of the Si-HA coatings were studied in comparison with that of hydroxyapatite (HA) coating prepared in the same way. The prepared Si-HA coatings and HA coating were only partially crystallized or in nano-scaled crystals. The introduction of Si element in HA significantly reduced P and Ca content, but densified the coating. The atom ratio of Ca to (P + Si) in the Si-HA coatings was in a range of 1.61–1.73, increasing slightly with an increase in the Si content. FTIR results displayed that Si entered HA in a form of SiO₄ unit by substituting for PO₄ unit. The cell attachment test showed that the HA and Si-HA coatings exhibited better cell response than the uncoated titanium, but no difference was observed in the cell response between the HA coating and the Si-HA coatings. Both the HA coating and the Si-HA coatings demonstrated a significantly higher cell growth rate than the uncoated pure titanium (p < 0.05) in all incubation periods while the Si-HA coating exhibited a significantly higher cell growth rate than the HA coating (p < 0.05). Si-HA with 0.42 at.% Si presented the best cell biocompatibility in all of the incubation periods. It was suggested that the synthesis mode of HA and Si-HA coatings in a simulated body environment in the biomimetic process contribute significantly to good cell biocompatibility.     

Investigation of La3 + Doped Yb2Sn2O7 as new thermal barrier materials

Low thermal conductivity is one of the key requirements for thermal barrier coating materials. From the consideration of crystal structure and ion radius, La^3 + Doped Yb₂Sn₂O₇ ceramics with pyrochlore crystal structures were synthesized by sol–gel method as candidates of thermal barrier materials in aero-engines. As La^3 + and Yb^3 + ions have the largest radius difference in lanthanoid group, La^3 + ions were expected to produce significant disorders by replacing Yb^3 + ions in cation layers of Yb₂Sn₂O₇. Both experimental and computational phase analyses were carried out, and good agreement had been obtained. The lattice constants of solid solution (La_xYb_1 − x)₂Sn₂O₇ (x = 0.3, 0.5, 0.7) increased linearly when the content of La^3 + was increased. The thermal properties (thermal conductivity and coefficients of thermal expansion) of the synthesized materials had been compared with traditional 8 wt.% yttria stabilized zirconia (8YSZ) and La₂Zr₂O₇ (LZ). It was found that La^3 + Doped Yb₂Sn₂O₇ exhibited lower thermal conductivities than un-doped stannates. Amongst all compositions studied, (La_0.5Yb_0.5)₂Sn₂O₇ exhibited the lowest thermal conductivity (0.851 W·m^− 1·K^− 1 at room temperature), which was much lower than that of 8YSZ (1.353 W·m^− 1·K^− 1), and possessed a high coefficient of thermal expansion (CTE), 13.530 × 10^− 6 K^− 1 at 950 °C.     

Enhanced thermal conductivity of Cu matrix composites reinforced with Ag-coated β-Si3N4 whiskers

Cu matrix composites reinforced with 10 vol.% Ag-coated β-Si₃N₄ whiskers (ASCMMCs) were prepared by powder metallurgy method. With the aim of improving the thermal conductivity of the composites, a quite thin Ag layer was deposited on the surface of β-Si₃N₄ whiskers. The results indicated that thermal conductivity of ASCMMCs with 0.30 vol.% Ag (0.30ASCMMCs) reached up to 273 W m⁻¹ K⁻¹ at 25 °C, which was 98 W m⁻¹ K⁻¹ higher than that of Cu matrix composites reinforced with uncoated β-Si₃N₄ whiskers (USCMMCs). The Ag coating could promote the densification of composites, reduce the aggregation of β-Si₃N₄ whiskers and enhance the Cu/Si₃N₄ interfacial bonding, therefore it could efficiently enhance the thermal conductivity of Cu matrix composites reinforced with β-Si₃N₄ whiskers (SCMMCs).