Novel Al2Mo3O12-PTFE composites for microwave dielectric substrates

Nowadays, microwave dielectric substrate materials have been extensively investigated to meet the requirements of rapid development in modern communications. Among them, the composites of ceramic powder filled polytetrafluoroethylene (PTFE) have been a hot topic. However, the compatibility and connectivity between the surface of ceramics and PTFE molecular chains in the samples are usually low. Herein novel PTFE based composites with different contents of Al₂Mo₃O₁₂ (20–60 wt%) modified by C₁₄H₁₉F₁₃O₃Si (F8261) coupling agent were designed and prepared. The coupling agent F8261 has been successfully grafted to the surface of Al₂Mo₃O₁₂ powders, effectively promoting the densification and dielectric properties of the composites. As the content of the modified Al₂Mo₃O₁₂ powders increases from 20 to 60 wt%, the ε_r value increases from 3.4 to 4.2, and tanδ almost remains constant at the beginning and increases with much more Al₂Mo₃O₁₂ added. The Al₂Mo₃O₁₂-PTFE composites filled with 30 wt% Al₂Mo₃O₁₂ present the optimal dielectric properties of ε_r = 3.6 and tanδ = 0.0018 with a high density of 95.6%. In addition, the electromagnetic and multiphysic simulation of a 24 GHz substrate integrated waveguide filter on the basis of the 30 wt% Al₂Mo₃O₁₂ - 70 wt% PTFE composite was carried out. It was revealed that the filter presented high stability on the electrical parameters caused by self-heating and dimension deformation due to the good microwave dielectric, thermal and mechanical properties of the substrate. These results indicate that the as-prepared 30 wt% Al₂Mo₃O₁₂ - 70 wt% PTFE composite would be a promising candidate for high-performance microwave dielectric substrates.     

Aerosol-deposited Al2O3/PTFE hydrophobic coatings with adjustable transparency

With the wide range of requirements for architectural glass, such as transparency, opacity, and hydrophobicity, there is a need to address the issues in the complexity of convention methods. Thus, considering functionality and applicability in various architectural windows, hydrophobic alumina/polytetrafluoroethylene (Al₂O₃/PTFE) composite layers with transparency or opacity were transferred to commercial architectural glass using a facile aerosol deposition (AD) process. We successfully fabricated hydrophobic coating layers with high transmittance (only a 0.03% difference from sheet glass) by optimizing the PTFE content in Al₂O₃ using solution-based synthesized powders to enable a uniform surface topology. The opaque hydrophobic Al₂O₃/PTFE coating layers exhibit a transmittance of approximately 0% with excellent hydrophobicity of 130°. Remarkably, this opaque film was successfully employed onto a large deposition area, curved substrate, and micro-patterned regions. It is believed that our AD-prepared composite layers have great potential for architectural glass in terms of economic feasibility and versatility.     

Preparation of in situ grown silicon carbide whiskers onto graphite for application in Al2O3-C refractories

C-SiC composite powders were prepared by salt-assisted synthesis from Si powders, graphite, and a molten salt medium (NaCl and NaF) with the molar ratio of Si/C =?1/2 at 1300?°C for 3?h. After the C-SiC composite powders part and complete replacement of the graphite, the mechanical properties, oxidation resistance and slag-corrosion resistance of the Al₂O₃-C materials were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as well as with dedicated equipment. The results indicated that SiC whiskers, with lengths of 10–50?nm, formed on the surface of the flake graphite, and the activation energy of oxidation of the C-SiC composite powder increased by 45.72?kJ?mol^?1 as compared to that of flake graphite. Furthermore, the decarburization area and slag erosion area of the Al₂O₃-C material decreased after 3?wt% of C-SiC composite powder was substituted for the flake graphite. Meanwhile, the cold modulus of rupture was maintained when 3?wt% of C-SiC composite powder was added. This improved both the oxidation and slag resistance of the Al₂O₃-C materials.     

Spark plasma sintering of Al-doped ZrB2–SiC composite

This research presents the influence of Al addition on microstructure and mechanical behavior of ZrB₂–SiC ultra-high temperature ceramic matrix composite (UHTCMC) fabricated by spark plasma sintering (SPS). A 2.5?wt% Al-doped ZrB₂–20?vol% SiC UHTCMC was produced by SPS method at 1900?°C under a pressure of 40?MPa for 7?min. The microstructural and phase analysis of the composite showed that aluminum-containing compounds were formed in-situ during the SPS as a result of chemical reactions between Al and surface oxide films of the raw materials (i.e. ZrO₂ and SiO₂ on the surfaces of ZrB₂ and SiC particles, respectively). The Al dopant was completely consumed and converted to the intermetallic Al₃Zr and Al₄Si compounds as well as Al₂O₃ and Al₂SiO₅. A relative density of 99.8%, a hardness (HV5) of 21.5?GPa and a fracture toughness (indentation method) of 6.3?MPa?m^1/2 were estimated for the Al-doped ZrB₂–SiC composite. Crack bridging, branching, and deflection were identified as the main toughening mechanisms.     

Effects of PTFE activation and carbon sources on combustion synthesis of Cr2AlC/Al2O3 composites

Formation of Cr₂AlC/Al₂O₃ in situ composites was investigated by self-propagating high-temperature synthesis (SHS) involving both PTFE activation and aluminothermic reduction. In addition to Al and Cr₂O₃ as the starting materials, carbon black, graphite, and Al₄C₃ were used as the carbon sources. PTFE was employed not only as a reaction promoter, but also as a carburizing agent. Depending on different sources of carbon, the threshold amounts of PTFE for inducing self-sustaining combustion were 1.5, 4.0, and 3.0 wt% for the samples adopting carbon black, graphite, and Al₄C₃, respectively. The combustion front velocity and temperature increased with increasing PTFE content. Moreover, the sample using carbon black was the most exothermic, while the Al₄C₃-based sample was the least. For the powder compacts adopting carbon black or graphite, Cr₂AlC/Al₂O₃ composites were produced with no impurities. Due to relatively weak reaction exothermicity, however, the synthesized composites containing small amounts of Cr₇C₃ and Al₄C₃ were obtained from the Al₄C₃-based reaction scheme.     

Hydrophobic composite coatings with photocatalytic self-cleaning properties by micro/nanoparticles mixed with fluorocarbon resin

A newly developed hydrophobic composite coating was fabricated by incorporating modified TiO₂ nanoparticles and hydrophobic material polytetrafluoroethylene (PTFE) micropowders dispersed in fluorocarbon resin. Moreover, the surface characteristics and self-cleaning properties of the newly developed composite material were examined. The material was found to exhibit sufficient hydrophobicity with a water contact angle of 133°. The surface free energy of the composite coating was 4.11 mJ/m². Scanning electron microscopy results revealed a micro/nanocomposite structure composed of PTFE micropowders and TiO₂ nanoparticles, which was verified by X-ray photoelectron spectroscopy results. Through ultraviolet irradiation the modified TiO₂-PTFE/FEVE composite coating successfully removed oleic acid absorbed on its surface. These results showed that the functional composite coating had a sufficiently hydrophobic surface with an efficient self-cleaning effect.     

Fabrication of BaTiO3-PTFE composite film for embedded capacitor employing aerosol deposition

The potential for using aerosol deposition (AD) as an alternative fabrication method to the conventional polymer composite process for embedded capacitors was examined. In order to achieve a high relative dielectric permittivity, BaTiO₃-polytetrafluoroethylene (PTFE) composite thick films were attempted by AD at room temperature. For the high dielectric constant, the BaTiO₃-PTFE composite films grown by AD should satisfied the following two critical conditions: a reduced decrement in ceramic particle size and a relieved distortion of the crystal structure. However, the relative permitivity of the composite films was too low compared with that of the BaTiO₃ films grown by AD. By predicting the dielectric constant in several composite models using the Hashin-Shtrikman bounds theory and 3-dimenstional (3-D) electrostatic simulation, we confirmed that the connectivity between ceramic particles is a highly critical factor for achieving a high dielectric constant in composite films.     

Tensile strength and corrosion resistance properties of porous Al2O3/Ni composites prepared with rice husk pore-forming agent

The mechanical performance and chemical stability of porous alumina materials operating under harsh service conditions are of utmost importance in understanding their operational behavior if they are to stand the test of time. In the present study, the joint effect of nickel (Ni) reinforcement and rice husk (RH) pore-forming agent (PFA) on the tensile strength and the corrosion resistance properties of composite porous alumina ceramics was studied. To exploit the potential of this new porous alumina system, plain and Ni-reinforced porous alumina samples (Al₂O₃-xNi-RH; x?=?2, 4, 6 and 8?wt%) were developed through the powder metallurgy technique. Comprehensive investigation on the tensile strength properties of the developed porous alumina ceramics showed that relative to the plain sample (tensile strength and elastic modulus; 6.1?MPa and 1201?MPa), the presence of highly stable Ni₃Al₂SiO₈ spinelloid promoted the tensile strength enhancement (12.6–6.4?MPa) and the elastic modulus decline (897–627?MPa) of the composite samples. Similarly, corrosion resistance test was performed on the composite porous alumina samples in both 10?wt% NaOH and 20?wt% H₂SO₄ hot aqueous solutions. Overall, the composite samples demonstrated superior chemical stability in NaOH solution as compared with the plain sample. On the other hand, the composites were more prone to attack in H₂SO₄ solution, except for the Al₂O₃-2Ni-10RH composite sample which maintained its superiority over the plain counterpart.     

Effects of Bi2O3-Nb2O5 additives on microstructure and magnetic properties of low-temperature-fired NiCuZn ferrite ceramics

NiCuZn ferrite with superior magnetic performance is vital ceramic material in multilayer chip inductors (MLCI) applications. In this study, low-temperature-sintered Ni_0.22Cu_0.2Zn_0.58Fe₂O₄ ferrite ceramic doped with 1.0?wt% Bi₂O₃-x?wt% Nb₂O₅ (where x?=?0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) was synthesized via solid-state reaction method. Effects of Bi₂O₃-Nb₂O₅ additives on microstructures and magnetic properties of NiCuZn ferrite ceramics sintered at 900?°C were systematically investigated. Results indicate that an appropriate amount of Bi₂O₃-Nb₂O₅ composite additives can significantly promote grain growth and densification of NiCuZn ferrite ceramics when sintered at low temperatures. Specifically, samples doped with 1.0?wt% Bi₂O₃ and 0.4?wt% Nb₂O₅ additives exhibited excellent initial permeability (~ 410 @ 1?MHz), high cutoff frequency (~ 10?MHz), high saturation magnetization (~ 54.92?emu/g), and low coercive force (~ 20.32?Oe). These observations indicate that NiCuZn ferrite ceramics doped with appropriate amounts of Bi₂O₃-Nb₂O₅ additives are great candidate materials for MLCI applications.     

Low-temperature pressureless sintering of Al2O3-SiC-Ni nanocermets in air environment

This paper introduces a simplified method for low-temperature pressureless sintering of Al₂O₃-Ni-SiC nanocermets in air environment. In this method, a thin and continuous Ni shell was coated on the surface of Al₂O₃ particles using electroless deposition method. The composite powders were subsequently compressed to prepare bulk specimens. By preventing the ceramic particles from direct contact during the densification of green specimens, sintering temperature of cermet materials was reduced from that of Al₂O₃ (>?1400?°C) to the range of Ni solid-phase sintering temperature. Furthermore, dissolution of a low amount of phosphorus in the composition of Ni coatings caused the further decrease of the sintering temperature to 800?°C. At such low temperatures, pressureless sintering of the cermets in the air environment was successfully performed instead of the common hot pressing process in a reducing atmosphere. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) characterizations indicated that the microstructure of such sintered samples consists of a continuous Ni network surrounding Al₂O₃ grains, without any structural defects or Ni oxidation. Furthermore, mechanical properties of the cermet materials were improved through reinforcement of the continuous Ni network by different amounts of SiC nanoparticles. The results showed that Al₂O₃-Ni-5?wt% SiC nanocermets sintered at 800?°C obtain the highest compressive strength of 242.5?MPa, hardness of 56.8 RA, and the lowest wear weight loss of 0.04?mg/m.     

Preparation of Al2O3-coated expanded graphite with enhanced hydrophilicity and oxidation resistance

Expanded graphite (EG), as a new kind of functional carbon-based material, is a vital supporting material and heat transfer enhancer for preparing highly conductive form-stable composite phase change materials (PCMs). However, the hydrophobic nature of EG makes it difficult to incorporate with inorganic PCMs. In this work, we intended to solve this drawback and a modified EG named Al₂O₃-coated EG which was characterized by enhanced hydrophilicity was developed via a heterogeneous nucleation technique and subsequent heat treatment. Experiments found that the Al₂O₃ layer on the surface of EG was uniform and essentially amorphous, and was well-bonded to EG via chemical interactions between oxygen atoms from Al₂O₃ and carbon atoms from EG. The hydrophilicity and oxidation resistance of Al₂O₃-coated EG could be enhanced by increasing the amount of Al₂O₃. Most importantly, compared with EG, the water contact angle of Al₂O₃-coated EG dropped from 90.7° to 33.9° when only 4.4?wt% Al₂O₃ was used, indicating that the hydrophilicity of EG could be greatly enhanced by low cost. Moreover, molecular dynamics (MD) simulation of the hydrophilicity of EG and Al₂O₃-coated EG proved that the preparation of Al₂O₃-coated EG was an efficient and feasible method to improve the hydrophilicity of EG.     

Effect of surface-modified Al2O3 on the thermomechanical properties of polybutylene succinate/Al2O3 composites

This study investigates the effect of the incorporation of alumina particles on the thermomechanical properties of polybutylene succinate (PBS)/Al₂O₃ composites. The alumina surface was modified with the carboxylic groups of maleic acid through simple acid-base and in situ polymerization reactions. Scanning electron microscope (SEM) results revealed the introduction of maleic acid treated alumina significantly affect the morphology of the PBS/Al₂O₃ composites as compared to the neat PBS. The thermal conductivity of the composite (0.411?W?m^?1 K^?1) was more than twice that of neat PBS. The composite containing polymerization-modified alumina showed a 50% increase in storage modulus compared with that of neat PBS. In addition, universal testing machine (UTM) and differential scanning calorimetry (DSC) measurements indicated an increase in the tensile strength and degree of crystallinity after the incorporation of modified alumina in the PBS/Al₂O₃ composite.     

Improved corrosion resistance of Al2O3 ceramic coatings on AZ31 magnesium alloy fabricated through cathode plasma electrolytic deposition combined with surface pore-sealing treatment

In this study, we explored the phase compositions and morphologies of the ceramic coatings from different aluminum sources (aluminum isopropoxide, aluminum nitrate, or a mixture of the two) prepared using cathode plasma electrolytic deposition (CPED) onto AZ31 magnesium alloys. Scanning electron microscopy and X-ray diffraction analyses of these coatings indicate that the deposited ceramic made from aluminum isopropoxide was composed of γ-Al₂O₃ whereas the one made from aluminum nitrate was composed of MgA1₂O₄, and that the former was more compact and uniform than the latter. A composite coating was prepared using epoxy resin as a protective layer that sealed the micropores on the CPED coating, thereby further improving its anticorrosion property. The elemental distribution of the cross-section of the composite coating was examined via energy dispersive spectroscopy. Corrosion resistance was investigated using potentiodynamic polarization curves and electrochemical impedance spectroscopy in a 3.5?wt% NaCl medium, and a salt spray test. The results indicate that the corrosion protection property of the Al₂O₃/epoxy resin coating of the magnesium alloy was better than that of the single Al₂O₃ coating. A cross-cut test revealed that the adhesion of the Al₂O₃/epoxy resin composite coating to the magnesium alloy surface was better than that of the single epoxy resin coating. The approach presented herein provides an attractive way to modify the surface of magnesium alloys to improve anticorrosion.     

Evaluation of hot corrosion behavior of CSZ,CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: usual CSZ, layer composite of CSZ/Micro Al₂O₃ and layer composite of CSZ/Nano Al₂O₃ in which Al₂O₃ was as a topcoat on CSZ layer. Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt% Na₂SO₄+55 wt% V₂O₅ molten salt at 1050 °C for 40 h. The graded microstructure of the coatings was examined using scanning electron microscope (SEM) and X-ray diffractometer (XRD) before and after hot corrosion test. The results showed that no damage and hot corrosion products was found on the surface of CSZ/Nano Al₂O₃ coating and monoclinic ZrO₂ fraction was lower in CSZ/Micro Al₂O₃ coating in comparison with usual CSZ. reaction of molten salts with stabilizers of zirconia (Y₂O₃ and CeO₂) that accompanied by formation of monoclinic zirconia, irregular shape crystals of YVO₄, CeVO₄ and semi-cubic crystals of CeO₂ as hot corrosion products, caused the degradation of CSZ coating in usual CSZ and CSZ/Micro Al₂O₃ coating.     

Fabrication and microstructure evolution of Al2O3/ZrBN-SiCN/Al2O3 high-temperature oxidation-resistant composite coating

The oxidation-resistance of thin film sensors, particularly at high temperatures, is critical for the lifetime and performance of the sensor. The preparation and oxidation-resistance of an Al₂O₃/ZrBN-SiCN/Al₂O₃ composite film with a sandwich-structure was performed using reactive magnetron sputtering. The microstructure evolution of the composite film is examined herein using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) analysis. Oxygen diffusion was significantly inhibited by the formation of crystalline Al₂SiO₅ and Zr-B-C amorphous phase inside the composite film. The Pt-13%Rh/Pt thin film thermocouple (TFTC) with the Al₂O₃/ZrBN-SiCN/Al₂O₃ composite film as a protective layer was fabricated and calibrated. Both the stability and lifetime of the TFTC was significantly enhanced for temperatures up to 1000?℃. The test error of the TFTC was reduced by half, compared with that of the TFTC with the Al₂O₃ protective layer, indicating an excellent oxidation-resistant performance of the composite film.     

Investigation of surface modification of rutile TiO2 nanoparticles with SiO2/Al2O3 on the properties of polyacrylic composite coating

Surface modification and characterization of TiO₂ nanoparticles as an additive in a polyacrylic clear coating were investigated. For the improvement of nanoparticles dispersion and the decreasing of photocatalytic activity, the surface of nanoparticles was modified with binary SiO₂/Al₂O₃. The surface treatment of TiO₂ nanoparticles was characterized with FTIR. Microstructural analysis was done by AFM. The size, particle size distribution and zeta potential of TiO₂ nanoparticles in water dispersion was measured by DLS method. For the evaluation of particle size and the stability of nanoparticles in water dispersions with higher solid content the electroacoustic spectroscopy was made. To determine the applicability and evaluate the transmittance of the nano-TiO₂ composite coatings UV–VIS spectroscopy in the wavelength range of 200–800 nm was employed. The results showed that surface treatment of TiO₂ nanoparticles with SiO₂/Al₂O₃ improves nanoparticles dispersion and UV protection of the clear polyacrylic composite coating.     

Growth of Al2O3–PTFE composite film at room temperature by aerosol deposition method

To fabricate a ceramic-based substrate for 3-dimensional integration modules with a thick film coating process at room temperature, aerosol deposition method was employed. Al₂O₃ was chosen as a main coating material for the requirements of low permittivity and dielectric loss. Especially to give a functionality of plasticity, composite film with polytetrafluoroethylene (PTFE) was also studied. The effects of PTFE, which was incorporated in the film, were investigated by the microstructural characterization. It was confirmed that Al₂O₃–PTFE film with the grain size of 100–200 nm were grown at room temperature using Al₂O₃–0.5 wt% PTFE mixture powders. Dielectric constant and dielectric loss of Al₂O₃–PTFE film were 4.5 and 0.005 at 1 MHz, respectively.     

Effects of interlayer roughness on deposition rate and morphology of aerosol-deposited Al2O3 thick films

The influence of the surface roughness of Al₂O₃ interlayers on the growth of Al₂O₃ thick films fabricated by an aerosol deposition (AD) process was investigated as an approach to improving the plasma resistance of the films. The Al₂O₃ interlayer was fabricated by a plasma electrolytic oxide (PEO) method. This method is capable of fabricating films on the entire surface area of 3-dimensional substrates, whereas the AD process has difficulties with depositing films on complex shapes, such as on edges and corners, and inside holes. To prevent degradation of the plasma resistance with increasing working time, the thickness of the Al₂O₃ interlayer was increased by the PEO method. The surface roughness of the Al₂O₃ interlayer was increased linearly by increasing the thickness of the Al₂O₃ interlayer. On Al₂O₃ interlayers with surface roughness values of more than 1.5 μm (R_a), Al₂O₃ films were not grown by the AD process. To investigate the effect of the surface roughness of the Al₂O₃ interlayer on the growth of Al₂O₃ films on the Al₂O₃ interlayer, we attempted to deposit Al₂O₃ films on an Al₂O₃ interlayer whose surface roughness was decreased from 1.5 μm to 0.8 μm by polishing. As a result, an Al₂O₃ film of 2.0 μm in thickness was grown by the AD process. These study results support the conclusion that controlling of the surface roughness is the most important factor in aerosol-deposited film growth.     

In-situ formation of Al2O3/Al core-shell from waste material: Production of porous composite improved by graphene

Aluminum dross produced from aluminum industry was used to fabricate Al₂O₃/Al porous composites. The dross was milled for 20?h to obtain nano powder. The milled material was examined by TEM and XRD. Graphene (up to 4?wt%) was mixed with the dross and utilized to reinforce sintered composites. The milled powders were compacted then fired at various temperatures up to 700?°C. Physical properties in terms of bulk density and apparent porosity for sintered composites were tested using Archimedes method. SEM attached by energy dispersive spectrometer (EDS) was used to inspect microstructure and elemental analysis of sintered composites. Microhardness and compressive strength were also measured. Ultrasonic nondestructive technique was utilized to examine the elastic moduli. Electrical conductivity of sintered composite was also studied. During milling up to 20?h, Al₂O₃/Al core-shell was in-situ formed with size of 65.9 and 23.8?nm, respectively. The apparent porosity of sintered composites was improved with rising graphene percent while it decreased with increasing sintering temperature. Increasing of graphene mass percent and firing temperature led to remarkable increase in all mechanical properties and electrical conductivity. The maximum compressive strength, hardness, elastic modulus and electrical conductivity were 200?MPa, 1200?MPa, 215?GPa and 1.42?×?10^?5 S/m, respectively, obtained for composite sintered at 700?°C having 4?wt% graphene.     

Composite Ni/Al2O3 coatings and their corrosion resistance

Composite coatings Ni/Al₂O₃ were electrochemically deposited from a Watts bath. Al₂O₃ powder with particle diameter below 1 μm was codeposited with the metal. The obtained Ni/Al₂O₃ coatings contained 5-6% by weight of corundum. The structure of the coatings was examined by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure, increasing its microcrystallinity and surface roughness. DC and AC electrochemical tests were carried out on such coatings in a 0.5 M solution of Na₂SO₄ in order to evaluate their corrosion resistance. The potentiodynamic tests showed that the corrosion resistance of composite coating Ni/Al₂O₃ is better than that of the standard nickel coating. After 14 days of exposure the nickel coating corrodes three times faster than the Ni/Al₂O₃ coating. The electrochemical behaviour of the coatings in the corrosive solution was investigated by electrochemical impedance spectroscopy (EIS). An equivalent circuit diagram consisting of two RC electric circuits: one for electrode, nickel corrosion processes and the other for processes causing coating surface blockage, were adopted for the analysis of the impedance spectra. The changes in the charge transfer resistance determined from the impedance measurements are comparable with the changes in corrosion resistance determined from potentiodynamic measurements.