Microstructural and thermochemical properties of Al/AlN/CuAl2 composite prepared by a combination of combustion synthesis and spark plasma sintering

A combination of combustion synthesis (CS) and spark plasma sintering (SPS) technology was employed in the fabrication of Al/AlN/CuAl₂ dense composites. Al/AlN/CuAl₂ composite powders in which a portion of the AlN was present in macro- and nanofiber forms were prepared by combustion of Al–Cu–5 wt.% (C₂F₄)_n, under a nitrogen atmosphere. The resulting composite powders were then subjected to consolidation by SPS at a dwell temperature level of 1500 °C, mechanical pressure of 60 MPa, and a non-isothermal heating time of 10 min. It is found that the actual thermal conductivity of Al/AlN/CuAl₂ composites fabricated with 5 wt.% (C₂F₄)_n is much higher than that of materials prepared in the absence (C₂F₄)_n. Maximum thermal conductivity (320 W/m K) was recorded for the samples prepared from an 0.8Al–0.2Cu–5 wt.% (C₂F₄)_n mixture. The influence of (C₂F₄)_n on the growth mechanism of AlN fibers and thermal conductivity of composite samples is discussed in light of the experimental data.     

The effects of pre-oxidation on the sintering and mechanical property of powder injection moulded SiC material

Usually, injection moulded SiC green parts are debound in inert atmosphere or vacuum, which induces the residual carbon and increases forming cycle and production cost. In this paper, injection moulded SiC with Al₂O₃ and Y₂O₃ as sintering assistant was thermal debound in air and Ar, respectively. The paper investigates the effects of pre-oxidation during debinding stage on the sintering and mechanical property of SiC material. During sintering, the oxide SiO₂ is in favour of the shrinkage of debound samples at lower temperature. After sintering, the linear shrinkage of sintered samples with pre-oxidation is bigger than the sample without pre-oxidation. Test results by TEM and XRD indicate that SiO₂ disappear from the inside of the sintered samples. The loss of SiO₂ decreases the content of Al₂O₃, which affects the formation of YAG (Y₃Al₅O₁₂). Sintered Sic samples contain α-SiC phase and intergranular phase. There is no hetero-phase between the boundaries of α-SiC phase and intergranular phase. The bending and compression strength values of sintered samples with pre-oxidation reach to 537 MPa and 2.89 GPa, respectively. These values approach the strength of sintered samples without pre-oxidation (594 MPa and 3.0 GPa).     

Metal injection molding of pure molybdenum

In this work the processing steps for producing molybdenum parts by means of metal injection molding technique, including milling, mixing, debinding and sintering, were investigated in detail. Different feedstocks were prepared from received and milled molybdenum powder and a thermoplastic binder based on paraffin wax. The feedstock including 50% by volume of milled molybdenum powder had better rheological property and was injected successfully at low pressure. The molded specimens were debinded by two different processes, direct thermal debinding and solvent–thermal debinding. The content of carbon and oxygen were respectively less than 0.060% and 0.0042% after the parts were sintered under hydrogen atmosphere. The parts sintered at 1850 °C for 120 min exhibited a highest density of 9.70 g/cm³,meanwhile the tensile strength and hardness was 229 MPa and 193 HV10, respectively.     

Low-firing of BiSbO4 microwave dielectric ceramic with V2O5–CuO addition

Effects of 1.0 wt.% V₂O₅–CuO mixture addition on the sintering behavior, phase composition and microwave dielectric properties of BiSbO₄ ceramics have been investigated. BiSbO₄ ceramics can be well densified below temperature about 930 °C with 1.0 wt.% V₂O₅–CuO mixtures addition with different ratios of CuO to V₂O₅. The formation of BiVO₄ phase and substitution of Cu²⁺ can explain the decrease of sintering temperature. Dense BiSbO₄ ceramics sintered at 930 °C for 2 h exhibited good microwave dielectric properties with permittivity between 19 and 20.5, Qf values between 19,000 and 40,000 GHz and temperature coefficient of resonant frequency shifting between ?71.5 ppm °C^?1 and ?77.8 ppm °C^?1. BiSbO₄ ceramics could be a candidate for microwave application and low temperature co-fired ceramics technology.     

High conductive ethylene vinyl acetate composites filled with reduced graphene oxide and polyaniline

A conductive network composed of reduced graphene oxide (RGO) planes and polyaniline (PANI) chains was designed and fabricated by in situ polymerization of aniline monomer on the RGO planes. It was further used for fabrication of conductive composites with a polymer matrix–ethylene vinyl acetate (EVA). The composites achieve improved conductivity at a low filler loading although the host polymer–EVA–is of insulator. For instance, compared to the pure EVA polymer, the conductivity of the composite filled with 4.0 wt.% RGO and 8.0 wt.% PANI increases from 1.2 × 10^?14 S cm^?1 to 1.07 × 10^?1 S cm^?1. In addition, thermal stability of the composites is also enhanced by the filler loading.     

Synergistic effect of boron nitride flakes and tetrapod-shaped ZnO whiskers on the thermal conductivity of electrically insulating phenol formaldehyde composites

Tetrapod-shaped zinc oxide (T-ZnO) whiskers and boron nitride (BN) flakes were employed to improve the thermal conductivity of phenolic formaldehyde resin (PF). A striking synergistic effect on thermal conductivity of PF was achieved. The in-plane thermal conductivity of the PF composite is as high as 1.96 W m⁻¹ K⁻¹ with 30 wt.% BN and 30 wt.% T-ZnO, which is 6.8 times higher than that of neat PF, while its electrical insulation is maintained. With 30 wt.% BN and 30 wt.% T-ZnO, the flexural strength of the composite is 312.9% higher than that of neat PF, and 56.2% higher that of the PF composite with 60 wt.% BN. The elongation at break is also improved by 51.8% in comparison with that of the composite with 60 wt.% BN. Such a synergistic effect results from the bridging of T-ZnO whiskers between BN flakes facilitating the formation of effective thermal conductance network within PF matrix.     

A novel fiber-reinforced polyethylene composite with added silicon nitride particles for enhanced thermal conductivity

A novel thermally conductive plastic composite was prepared from a mixture of silicon nitride (Si₃N₄) filler particles and an ultrahigh molecular weight polyethylene–linear low density polyethylene blend. The effects of Si₃N₄ particle sizes, concentration, and dispersion on the thermal conductivity and relevant dielectric properties were investigated. With proper fabrication the Si₃N₄ particles could form a continuously connected dispersion that acted as the dominant thermally conductive pathway through the plastic matrix. By adding 0–20% Si₃N₄ filler particles, the composite thermal conductivity was increased from 0.2 to ～1.0 W m^?1 K^?1. Also, the composite thermal conductivity was further enhanced to 1.8 W m^?1 K^?1 by decreasing the Si₃N₄ particle sizes from 35, 3 and 0.2 μm, and using coupling agent, for the composites with higher filler content. Alumina short fibers were then added to improve the overall composite toughness and strength. Optimum thermal, dielectric and mechanical properties were obtained for a fiber-reinforced polyethylene composite with 20% total alumina–Si₃N₄ (0.2 μm size) filler particles.     

Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

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.     

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.     

Porous high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2: A novel strategy towards making ultrahigh temperature ceramics thermal insulating

Transition metal diborides based ultrahigh temperature ceramics (UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity arises from both electronic and phonon contributions. Thus electronic and phonon contributions must be controlled simultaneously in reducing the thermal conductivity of transition metal diborides. In high entropy (HE) materials, both electrons and phonons are scattered such that the thermal conductivity can significantly be reduced, which opens a new window to design novel insulating materials. Inspired by the high entropy effect, porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ is designed in this work as a new thermal insulting ultrahigh temperature material and is synthesized by an in-situ thermal borocarbon reduction/partial sintering process. The porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ possesses high porosity of 75.67%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low room temperature thermal diffusivity and thermal conductivity of 0.74 mm² s⁻¹ and 0.51 W m⁻¹ K⁻¹, respectively. In addition, it exhibits high compressive strength of 3.93 MPa. The combination of these properties indicates that exploring porous high entropy ceramics such as porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ is a novel strategy in making UHTCs thermal insulating.     

Effects of CaF2 addition on sintering behavior and microwave dielectric properties of ZnTa2O6 ceramics

Microwave dielectric ceramics ZnTa₂O₆ were prepared by conventional mixed oxide route. The effects of CaF₂ addition on the microstructures and microwave dielectric properties of ZnTa₂O₆ ceramics were investigated. Formation of second phase can be detected at the high addition of CaF₂ (0.5–1.0 wt.%). Variation of grain shapes were observed with CaF₂ content increasing. The sintering temperature of CaF₂-doped ZnTa₂O₆ ceramics can be effectively lowered from 1400 °C to 1225 °C due to liquid phase effect. The microwave dielectric properties were affected by the amount of CaF₂ addition. At 1225 °C for 4 h, ZnTa₂O₆ ceramics with 0.25 wt.% CaF₂ possesses excellent microwave dielectric properties: ε_r = 31.32, Q × ? = 73600 GHz(6.8 GHz) and τ_? = ? 6.97 ppm/°C.     

Hydrothermal synthesis of mesoporous VO2·½(H2O) nanosheets and study of their electrical properties

Layered sheet-like nanocrystalline VO₂·½(H₂O) has been synthesized by hydrothermal process using V₂O₅ as vanadium source and 2-phenylethylamine as a reducing agent and a structure-directing template. Techniques X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and nitrogen adsorption/desorption isotherms have been used to characterize the structure, morphology and composition of the materials. Electrical conductivity measurements showed that the as synthesized VO₂·½(H₂O) nanosheets has a conductivity value which goes from 75 × 10^?6 Ω^?1 cm^?1 at 298 K, to 68 10^?5 Ω^?1 cm^?1 at 386 K with activation energy of 0.24 eV.     

An innovative process to fabricate copper/diamond composite films for thermal management applications

Diamond dispersed copper matrix (Cu/D) composite films with strong interfacial bonding were produced by tape casting and hot pressing without carbide forming additives. The tape casting process offers an original solution to obtain laminated materials with accurate thickness control, smooth surface finish, material net-shaping, scalability, and low cost. This study presents an innovative process of copper submicronic particles deposition onto diamond reinforcements prior to densification by hot pressing. Copper particles act as chemical bonding agents between the copper matrix and the diamond reinforcements during hot pressing, thus offering an alternative solution to traditionnal carbide-forming materials in order to get efficient interfacial bonding and heat-transfer in Cu/D composites. It allows high thermal performances with low content of diamond, thus enhancing the cost-effectiveness of the materials. Microstructural study of composites by scanning electron microscopy (SEM) was correlated with thermal conductivity and thermal expansion coefficient measurements. The as-fabricated films exhibit a thermal conductivity of 455 W m^?1 K^?1 associated to a coefficient of thermal expansion of 12 × 10^?6 °C^?1 and a density of 6.6 g cm^?3 with a diamond volume fraction of 40%, which represents a strong enhancement relative to pure copper properties (λ_Cu = 400 W m^?1 K^?1, α_Cu = 17 × 10^?6 °C^?1, ρ_Cu = 8.95 g cm^?3). The as-fabricated composite films might be useful as heat-spreading layers for thermal management of power electronic modules.     

Polysaccharidic binders for the conception of an insulating agro-composite

The objective of this study is the formulation of a natural polysaccharidic binder for the conception of an insulating bio-based composite made with sunflower stalk particles. The formulation was performed using chitosan cross-linked with Genipin and mixed with alginate, guar gum and starch. A fractional factorial experimental design within 32 essays was established to find the formulation leading to composites with the best combination between good mechanical properties and limited amount of chitosan in the binder. Composites with a thermal conductivity (κ) of 0.07 W m⁻¹ K⁻¹ and a maximum tensile stress (σ_max) of 0.2 MPa were obtained with a total binder ratio of 5.5% (w/w). The results of this study show that the insulating bio-based composites evaluated have competitive mechanical and thermal performances compared with other eco-friendly insulating materials available on the market.     

Hydrothermal synthesis,characterization and electrical investigation of poly(para-phenylenediamine)/vanadium oxide nanocomposite nanosheets

Sheet-like mesoporous poly(paraphenylenediamine)/vanadium oxide nanocomposite has been synthesized by the hydrothermal process using the vanadium oxide V₂O₅ as inorganic precursor and paraphenylendiamine as reducing and a structure-directing agent. Such techniques as X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal analysis (TG-DTA), Fourier transform infrared spectroscopy (FTIR), Raman Spectroscopy, X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption/desorption isotherms (BET) have been used to characterize the structure, morphology and the texture of the material. The conductivity of the material was measured by complex impedance spectroscopy which increases from 72 × 10^?5 Ω^?1 cm^?1 at 298 K to 95 × 10^?4 Ω^?1 cm^?1 at 493 K. The Arrhenius diagram is not linear, it presents a rupture situated at 407 K and the activation energies’ average values are 0.044 eV and 0.13 eV.     

The thermoelectric properties of InxCo4Sb12 alloys prepared by HPHT

In_xCo₄Sb₁₂ (0.1 ≤ x ≤ 0.5) skutterudite compounds with bcc crystal structure have been prepared by high-pressure and high-temperature (HPHT) method. Through this method, the processing time can be reduced from a few days to half an hour. The constituent phases of all resultant samples were determined by X-ray diffraction. The Seebeck coefficient, electrical resistivity, power factor and thermal conductivity of In_xCo₄Sb₁₂ (0.1 ≤ x ≤ 0.5) were all measured in the temperature range of 302–665 K. Among all the samples, In_0.5Co₄Sb₁₂ showed the highest power factor of 31.3 μWcm^? 1 K^? 2 at 616 K and the lowest thermal conductivity of 2.193 Wm^? 1 K^? 1 at 568 K. As a result, the maximum dimensionless figure of merit (ZT) value of In_0.5Co₄Sb₁₂ reached 0.88 at 665 K, which can be attributed to its low thermal conductivity and high power factor.     

Synthesis of lanthanum zirconium oxide nanomaterials through composite-hydroxide-mediated approach

A novel thermal barrier coating material, lanthanum zirconium oxide (La₂Zr₂O₇) has been synthesized through the composite-hydroxide-mediated method at low temperature. The phase structures, morphology, thermal stability and thermal conductivity of the as-synthesized La₂Zr₂O₇ were investigated systematically. The X-ray diffraction (XRD) patterns revealed a single phase with cubic pyrochlore structure for La₂Zr₂O₇ after treated at 1300 °C for 100 h. The transmission electron microscope (TEM) and scanning electron microscope (SEM) analyses showed that the sample was made up of sphere-like nanoparticles with the size between 50 and 100 nm. Furthermore, the thermal analysis result demonstrated the La₂Zr₂O₇ sample had high thermal stability even at 1300 °C. As the temperature increased to 1200 °C, the thermal conductivity value could be as low as 1.75 W m^?1 K^?1. Due to the high-temperature stability and lower thermal conductivity, the La₂Zr₂O₇ material is expected to be a promising candidate for the use of thermal barrier coatings.     

Preparation,structure and electrical conductivity of pyrochlore-type samarium–lanthanum zirconate ceramics

A series of zirconate compounds with the general formula Sm_2–xLa_xZr₂O₇ (0 ? x ? 1.0) were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, structure and electrical conductivity of Sm_2–xLa_xZr₂O₇ ceramics were investigated by the Archimedes method, X-ray diffraction and impedance spectroscopy measurements. Sm_2–xLa_xZr₂O₇ (0 ? x ? 1.0) ceramics exhibit a pyrochlore-type structure. The measured electrical conductivity of Sm_2–xLa_xZr₂O₇ ceramics obeys the Arrhenius relation and gradually increases with increasing temperature from 673 to 1173 K. Sm_2–xLa_xZr₂O₇ ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10^?4 to 1.0 atm at all test temperature levels. The electrical conductivity of Sm_2–xLa_xZr₂O₇ ceramics decreases with increasing lanthanum content at identical temperature levels.     

Flux growth and thermal properties of LiBaB9O15 single crystal

A large LiBaB₉O₁₅ single crystal has been grown by the top-seeded solution growth (TSSG) method using a Li₂Mo₃O₁₀ flux system. The crystal obtained exhibits (1 1 0), (1 1 3) and (1 0 2) faces. For the first time, thermal properties of the as-grown crystal, including thermal expansion, specific heat and thermal conductivity, have been investigated as a function of temperature. The specific heat of the LiBaB₉O₁₅ crystal was measured to be 0.663–1.110 J g^?1 K^?1 over the temperature range of 20–400 °C. The crystal exhibits thermal expansion along the a- and b-axis, coupled with thermal contraction along the c-axis, over the measured temperature range of 25–500 °C. The average thermal expansion coefficients along the a- and c-axis of the LiBaB₉O₁₅ crystal from 25 to 500 °C are calculated to be α_a = 6.56 × 10^?6 K^?1 and α_c = ?4.82 × 10^?6 K^?1, respectively.