Search

Understanding the causal relations between the load situation and the lifetime at high temperature is extremely important in the application of alumina foams in high temperature industry. In this work, alumina foams were obtained at the sintering temperature of 1700 °C. The observation of microstructure of alumina foams showed that the structure of grains in the pore wall were of point contact structure. Key properties, such as flexural strength, elastic modulus, compressive creep, coefficient of thermal expansion and thermal conductivity of alumina foams were measured from room temperature to 1400 °C. The variation of the above properties of alumina foams as a function of temperature was investigated. The results showed that significant decrease in flexural strength and elastic modulus were observed when the temperature was over 1000 °C. Under this unique structure, minor liquid phase at the grain boundary could lead to the grain slip and further the remarkable decreases of the high temperature performance.

Particle size reduction of Y3Al5O12:Ce (YAG:Ce) phosphor is highly needed for Micro-LED display applications. In this work, size control of Y3Al5O12:Ce phosphor particles is achieved via carbon coating and further heat treatment. A thin layer of carbon is deposited on the surface of Y3Al5O12:Ce by chemical vapor deposition (CVD). During the heat treatment, carbon reacted with oxygen element in the phosphor and escape from phosphor particles. The reaction results in the phosphor breaking into smaller particles. The phosphors are characterized by laser light scattering for particle size distribution, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for morphologies, XRD refinements for crystal structure, electron energy-loss spectroscopy (EELS) for elemental analysis. It is demonstrated that the median diameter (D50) of the phosphor particle size has been significantly reduced from 19.1 to 0.96 μm yet the photoluminescent properties have little changes. The carbon coating and further heat treatment method shows potential application in size control of phosphors.

Polymer-derived ceramics demonstrate great potential as lithium-ion battery anode materials with good cycling stability and large capacity. SiCNO ceramic nanoparticles are produced by the pyrolysis of polysilazane nanoparticles that are synthesized via an oil-in-oil emulsion crosslinking and used as anode materials. The SiCNO nanoparticles have an average particle size of around 9 nm and contain graphitic carbon and Si3N4 and SiO2 domains. Composite anodes are produced by mixing different concentrations of SiCNO nanoparticles, edge-functionalized graphene oxide, polyvinylidenefluoride, and carbon black Super P. The electrochemical behavior of the anode is investigated to evaluate the Li-ion storage performance of the composite anode and understand the mechanism of Li-ion storage. The lithiation of SiCNO is observed at ∼0.385 V versus Li/Li+. The anode has a large capacity of 705 mA h g-1 after 350 cycles at a current density of 0.1 A g-1 and shows an excellent cyclic stability with a capacity decay of 0.049 mA h g-1 (0.0097%) per cycle. SiCNO nanoparticles provide a large specific area that is beneficial to Li+ storage and cyclic stability. In situ transmission electron microscopy analysis demonstrates that the SiCNO nanoparticles exhibit extraordinary structural stability with 9.36% linear expansion in the lithiation process. The X-ray diffraction and X-ray photoelectron spectroscopy investigation of the working electrode before and after cycling suggests that Li+ was stored through two pathways in SiCNO lithiation: (a) Li-ion intercalation of graphitic carbon in free carbon domains and (b) lithiation of the SiO2 and Si3N4 domains through a two-stage process.

Manganese zinc (Mn-Zn) ferrites were prepared by solid-state reaction. The sintering process was investigated in detail using differential scanning calorimetry/thermogravimetric analysis and x-ray diffraction characterizations. A three-step weight loss was observed and the main Mn-Zn ferrite formation reaction was detected. Two-step heat treatment was employed to achieve high density Mn-Zn ferrite. For sintering temperatures from 1300°C to 1400°C, crystal grains of Mn-Zn ferrite became uniform with larger size and more equiaxed shape. The grain boundaries changed from curved to straight. Such features were believed helpful for the magnetic domain wall movement. In addition, better diffusivity at higher sintering temperature homogenized the distribution of impurities which also facilitated the magnetic domain alignment. Thanks to the microstructure acquired by controlling the sintering process, a high-amplitude permeability of 1151, a low total loss of 411 W/kg and coercivity of 57.63 A/m at 65 kHz were simultaneously achieved on samples sintered at 1400°C.

In this work, the microstructure, crystallographic parameters and magnetic properties of polycrystalline manganese–zinc (Mn–Zn) ferrites were investigated. Mn1−xZnxFe2O4 (0.11 ≤ x ≤ 0.14) were prepared by the solid-state reaction method at 1360 °C for 2 h in air atmosphere. Phase evolution and microstructure of Mn–Zn ferrites were investigated by XRD and SEM techniques. X-ray diffraction study confirmed the formation of spinel-structured Mn1−xZnxFe2O4. The magnetic properties were measured employing soft magnetic alternating current measuring instrument (MATS-3010SA). It was observed that as the zinc content (x) increases, the lattice constant remained stable, microstructure did not change significantly, the permeability of Mn–Zn ferrite was improved, and total loss and coercivity were reduced. Changes in crystal structure and magnetism were attributed to occupancy of non-magnetic Zn2+ and super-exchange interactions between the metal ions of sub-lattice A and B in the spinel structure. These results indicated that the magnetic properties of Mn1−xZnxFe2O4 ferrites were strongly affected by the Zn content (x). In a brief summary, Mn0.86Zn0.14Fe2O4 ferrite exhibited excellent magnetic properties with a low loss (Ps = 39.24 W/kg) and low coercivity (Hc = 39.16 A/m) measured at 200 mT and 10 kHz.

Low infrared emissivity in 3–5 μm or 8–14 μm band has been realized in different host materials. Realizing the low infrared emissivity in single material covering the entire band is rarely achieved. This research explores a binary system of ultra-high temperature ceramic ZrB2–CeO2 composites prepared by solid state sintering. their phase composition, microstructure evolution, reaction mechanism and infrared emissivity performance were discussed in detail. It was found that CeO2, ZrB2, ZrO2, and ZrnCe1-nO2 existed in the sample in addition to the coated glass phase of CeO2 on the surface of the sample. In-situ HRTEM results clearly revealed a series of reaction mechanism of CeO2 and ZrB2. With the increase of sintering temperature, the CeO2 particles gradually transformed into a glass phase, and were distributed on the surface of ZrB2 particles, and ZrO2 was formed on the surface of ZrB2, simultaneously. Consequently, ZrnCe1-nO2 is formed as the reaction of ZrO2 and CeO2. Compared with the infrared emissivity spectra of pure CeO2 and ZrB2, a low infrared emissivity less than 0.2 in a wider coverage band of 3–5 and 8–14 μm in ZrB2–CeO2 compounds was achieved as a result of synergistic interaction between CeO2 and ZrB2. The obtained information about the phase composition and microstructure evolution helps to realize low infrared emissivity in a wider coverage band in compounds via composition and microstructure modification.

K2SiF6:Mn4+ phosphor is well known for its excellent red emission performance which is vital for improving the color rendering of white light-emitting diodes. However, the poor moisture resistance limits its application in optical devices. In this paper, K2SiF6:Mn4+ phosphor is coated with an inorganic hydrophobic protective layer to obtain good moisture resistance. Chemical vapor deposition method was used to decompose acetylene at high temperature, and the generated nanoscale carbon layer worked as a hydrophobic protective coating on the surface of the phosphor. Microstructure, compositions and properties of the synthesized K2SiF6:Mn4+@C phosphor were investigated in detail. It is found that most of the deposited carbon is coated on the surface of phosphor crystals in amorphous state. The carbon atoms are bonded with the fluorine element in K2SiF6:Mn4+ phosphor, forming carbon-fluorine (C–F) covalent bonds. The moisture resistance of K2SiF6:Mn4+@C phosphor is improved owing to the protection of the hydrophobic carbon. The relative emission intensity of K2SiF6:Mn4+@C phosphor could maintain 73% of the initial luminous intensity after immersing in the aqueous solution at room temperature for 8 h, whereas K2SiF6:Mn4+ phosphor without carbon coating was only 0.7% remaining of the initial value under the same conditions.

Negative permittivity is a required physical property of metamaterials which has been widely used in some emerging and unconventional applications, such as cloaking, perfect lens and perfect wave absorption. In addition to periodic meta-structured units, a composite with conductive networks, so-called metacomposite, is equivalent to diluted metals, which also exhibits negative permittivity properties at microwave frequency. The percolative metacomposites have the advantages of isotropic property, flexible preparation methods and low cost in achieving negative permittivity. However, building a percolation network from conductive fillers in an insulating matrix is a complicated process, and the dispersion of fillers is difficult to control. In this study, the percolation network was modulated on the basis of the conductive copper foam by anodization treatment. The conductivity and negative permittivity of the foam varied with the anodization time. As the anodization time increased, the foam changed from a typical Drude metal to a less conductive 3D structure with mitigated negative permittivity. The absolute value of the negative permittivity of Cu-30V-5h sample was two orders of magnitude lower than that of the samples anodised for 30V-3h, 20V-3h and 20V-1h. Therefore, anodization is a facile and effective method to control the conductivity and negative permittivity of copper epoxy metacomposites.

Atmospheric nitrogen fixation using a photocatalytic system is a promising approach to produce ammonia. However, most of the recently explored photocatalysts for N2 fixation are in the powder form, suffering from agglomeration and difficulty in the collection and leading to unsatisfactory conversion efficiency. Developing efficient film catalysts for N2 photofixation under ambient conditions remains challenging. Herein, we report the efficient photofixation of N2 over a periodic WS2@TiO2 nanoporous film, which is fabricated through a facile method that combines anodization, E-beam evaporation, and chemical vapor deposition (CVD). Oxygen vacancies are introduced into TiO2 nanoporous films through Ar annealing treatment, which plays a vital role in N2 adsorption and activation. The periodic WS2@TiO2 nanoporous film with an optimized WS2 content shows highly efficient photocatalytic performance for N2 fixation with an NH3 evolution rate of 1.39 mmol g−1 h−1, representing one of the state-of-the-art catalysts.

In this study, we report a nonprecious metal catalyst for high-efficiency hydrogen evolution reaction (HER). A self-organized S-doped MoP nanoporous layer (S-MoP NPL) is achieved through a facile e...

Understanding the synergism of bimetallic transition metal (TM)-based catalysts for oxygen evolution reaction (OER) is very difficult because it is complicated to identify the surface active sites in a bimetal system. Herein, we rationally designed Cu oxide (CuOx) nanoarray film (NF) as an example to investigate the synergism and doping effects of iron group metals on OER. This is an advantage because CuOx is electrocatalytically inert and oxidatively stable, which is much better than carbon-based platforms. Especially, cobalt (Co) shows a much stronger synergism as compared with nickel (Ni) and iron (Fe). By introducing Co into the inert CuOx NFs, the Co active sites can be correlated to the OER activity by rationally regulating the morphology of CuOx NFs. In addition, the phase transformation from Cu2O to CuO occurs during the OER testing, further boosting the OER activity of Co-doped CuOx NF due to the hybridization change of Co active site. As a result, the Co-doped CuOx NF with 0.30 at. % Co (denoted...

A hybrid ceramic-polymer composite is fabricated by a co-curing lay-up process by combining a carbon nanotube (CNT) reinforced ceramic composite thin film with a carbon fiber reinforced polymer (CFRP) composite substrate. The ceramic nanocomposite thin film has good flexibility, thermal conductivity and high temperature tolerance. The polymer composite substrate is a carbon fiber reinforced bismaleimide composite that is widely used in aerospace and automotive industries. Finite element analysis (FEA) is used to investigate the maximum survival temperature with different thicknesses of the ceramic nanocomposite. The resultant hybrid composite shows good structural integrity and displays a pull-off bonding strength up to 8.3 MPa. In addition, thermal study illustrates that such a flexible CNT reinforced ceramic composite can effectively protect CFRP in an elevated temperature environment by delaying transient thermal conduction.

In this paper, a novel three-dimensional-linked (3D-linked) carbon fiber-carbon nanotube (C f -CNT) hybrid structure is presented to improve the through-thickness thermal properties of polymer derived silicon carbonitride matrix composites. Comparing with pristine plain woven carbon fiber, the pores in the criss-cross position are fully filled with carbon nanotubes (CNT) in the presented C f -CNT hybrid structure. CNT can be integrated in between the plain woven carbon fibers and generate a 3D-linked C f -CNT hybrid network structure. Thanks to the percolating network of CNT and the densification effect, the through-thickness thermal conductivity of the resultant ceramic matrix composites is improved significantly, which is enhanced by 10% with short CNT and by 31% with long CNT, respectively. The thermal conductivity is found to increase with temperature from 100 to 900 °C for these composites. Multiphysics simulation is conducted to reveal the effects of the 3D-linked C f -CNT hybrid structure on the thermal conductivity at various temperatures, which are consistent with experimental results.

A lot of efforts have been devoted to improving the stability of phosphors. Due to the high stability and high transmittance of graphene material, we deposited a thin carbon layer on the surface of SrSi2O2N2: Eu2+ phosphor particles by chemical vapor deposition method as to improve its thermal and chemical stability. Chemical composition and microstructure of the coated particles were characterized and analyzed in detail. After coating, the carbon was attached on the phosphor in an amorphous form. The carbon coating alleviated the oxidation of Eu2+, which enhanced the luminous stability and extended the service time of the phosphors. The results showed that the carbon-coated phosphors better retained luminescence intensity at high temperatures. In addition, its oxidation resistance and thermal stability of fabricated LED had also been improved. Therefore, applying carbon coating on the phosphor is an effective way to improve the luminous stability of the phosphor.

Magnesium (Mg) and its alloys are among the lightest metallic structural materials, making them very attractive for use in the aerospace and automotive industries. Recently, Mg has been used in metal matrix composites (MMCs), demonstrating significant improvements in mechanical performance. However, the machinability of Mg-based MMCs is still largely elusive. In this study, Mg-based MMCs are machined using a wide range of cutting speeds in order to elucidate both the chip morphology and chip formation mechanism. Cutting speed is found to have the most significant influence on both the chip morphology and chip formation mechanism, with the propensity of discontinuous, particle-type chip formation increasing as the cutting speed increases. Saw-tooth chips are found to be the primary chip morphology at low cutting speeds (lower than 0.5 m/s), while discontinuous, particle-type chips prevail at high cutting speeds (higher than 1.0 m/s). Using in situ high speed imaging, the formation of the saw-tooth chip morphology is found to be due to crack initiation at the free surface. However, as the cutting speed (and strain rate) increases, the formation of the discontinuous, particle-type chip morphology is found to be due to crack initiation at the tool tip. In addition, the influences of tool rake angle, particle size, and particle volume fracture are investigated and found to have little effect on the chip morphology and chip formation mechanism.

Al/Fe2O3 thermite powders were prepared by cryomilling at liquid nitrogen temperature. The cryogenic temperature will restrain the mechanochemical reaction between alumina and iron oxide, leading to high reactivitive nanoscale powders. The size distribution of the powders was analyzed using laser particle size analyzer, and cryomilling was proved to be an effective method to prepare ultrafine powders. The differential scanning calorimetry (DSC) analysis indicated that the cryomilled powders get more fully-reacted, a larger proportion of solid-solid reaction and more heat release in the solid-liquid reaction, comparing with the powders milled at room temperaure. Furthermore, the reaction kinetics of Al-Fe2O3 system is analyzed by a model-free Starink method. The activation energy for solid-solid reaction of 2Al-Fe2O3 thermite mixture cryomilled for 40 min is determined as 250 kJ/mol. The alternating gradient magnetometer (AGM) analysis shows that long time milling evoked the thermit reaction between Al and Fe2O3, leading to the increase in saturation magnetization (Ms) and remanent magnetization (Mr).