The microstructure,coefficient of thermal expansion and flexural strength of cordierite ceramics prepared from alumina with different particle sizes

Cordierite ceramics were produced from alumina with 5 and 0.65 μm particle sizes or AlOOH and talc, clays and feldspar, to determine the influence of the alumina particle size on the microstructure, coefficient of thermal expansion (CTE) and flexural strength (FS) of the ceramics. After sintering at 1300 °C the ceramics made from 5-μm-sized alumina consisted of cordierite, glass, quartz, mullite and alumina, and had the highest density, FS and CTE. The alumina grains act as inclusions, from which the trajectories of the cracks were deflected or terminated, which increases the FS and CTE. The ceramics from sub-micrometre-sized alumina or AlOOH contained a negligable amount and no alumina, respectively, together with other phases. This is reflected in the low CTE and FS. The cordierite ceramic with the lowest CTE of ∼2.0 × 10⁻⁶ K⁻¹ and a high FS of 100 MPa was prepared from the 0.65-μm-sized alumina particles.     

Electrical and thermal properties of low permittivity Sr2Al2SiO7 ceramic filled HDPE composites

The feasibility of low permittivity Sr₂Al₂SiO₇ (SAS) ceramic filled high density polyethylene (HDPE) composites for substrate and packaging applications has been investigated in this paper. The composites were prepared by the melt mixing and hot pressing techniques. Scanning electron microscopic images of SAS filled HDPE showed the increased connectivity with filler loading. The composites showed excellent relative density (>98%) with low bulk density (<2.40 g cm^?3) and very low moisture absorption (<0.10 wt%). The relative permittivity (ε_r) and the dielectric loss (tan δ) at 1 MHz and at 5 GHz were found to be low and found to increase with filler volume fraction (V_f). The experimentally observed relative permittivity at 5 GHz was correlated with the values proposed by different theoretical models. Among them, effective medium theory (EMT) gave better fit with experimental values except at the highest filler loading (0.50 V_f). Improvement in the thermal properties was also observed with filler content. The coefficient of linear thermal expansion (CTE) was found to decrease with filler content. Thermal conductivity (TC) of the composite was greatly enhanced as a function of filler volume fraction. The composite with 0.50 filler volume fraction showed balanced thermal and dielectric properties with ε_r=4.2, tan δ=3.9×10^?3, TC=2.2 W m^?1 K^?1 and CTE=101 ppm/°C.     

Effect of SiCp multimodal distribution on pitting behavior of Al/SiCp composites prepared by reactive infiltration

The effect of coated-SiC_p multimodal-size-distribution on the pitting behavior of Al/SiC_p composites was investigated. α-SiC powders (10, 54, 86, and 146 μm) were properly mixed and coated with silica to produce porous preforms with 0.6 volume fraction of the reinforcement with monomodal, bimodal, trimodal, and cuatrimodal size distribution. The preforms were infiltrated with the alloy Al–13 Mg–1.8Si (wt.%) in argon followed by nitrogen at 1100 ºC for 60 min. The composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) before and after cyclic polarization measurements in 0.1 M NaCl de-aerated solutions. Results show that whereas corrosion and passivation potentials are not influenced with increase in SiC_p particle size distribution, favorably, the susceptibility to pitting corrosion decreases. This beneficial effect is ascribed to the smaller area of the alloy matrix exposed to the chloride solution with augment in particle size distribution, substantially when going from monomodal to bimodal SiC_p particle size distribution.     

Enhancement of the thermal stability and mechanical properties of a PMMA/aluminum trihydroxide composite synthesized via bead milling

The use of aluminum trihydroxide (ATH) fillers as non-halogen flame retardants for polymethylmethacrylates (PMMA) creates a conflict between the mechanical properties and heat resistance of the composites. Therefore, to ensure that the PMMA mechanical properties remain satisfactory, improvements in both the filler–polymer interactions and the ability to control the size and size distribution, morphology and dispersion of the fillers are required. Thus, in the present study, bead milling was used to control both the size distribution and dispersion of ATH fillers in MMA, which had an initial average size of 0.75 μm. The dispersion was obtained by alteration of the surface characteristics of ATH fillers using a silane-based dispersing agent, (3-acryloxypropyl) trimethoxysilane (APTMS). Bead milling successfully comminuted the ATH particles and prevented the formation of ATH agglomerates. The smallest average size of the ATH particles after bead milling was 300 nm. Highly dispersed ATH filler particles were observed in the TEM images of the PMMA/ATH composites. The filler–polymer interaction, i.e. the interaction parameter (B), was calculated. The effects of volume fraction, particle size distribution, and surface modification of the fillers on the results of dynamic mechanical analysis (DMA) are discussed. The thermal stability of the PMMA/ATH composites was also investigated using thermal gravimetric analysis (TGA).     

The hierarchical structure and properties of multifunctional carbon nanotube fibre composites

The axial mechanical, electrical and thermal properties of carbon nanotubes (CNTs) can be exploited macroscopically by assembling them parallel to each other into a fibre during their synthesis by chemical vapour deposition. Multifunctional composites with high volume fraction of CNT fibres are then made by direct polymer infiltration of an array of aligned fibres. The fibres have a very high surface area, causing the polymer to infiltrate them and resulting in a hierarchical composite structure. The electrical and thermal conductivities of CNT/epoxy composites are shown to be superior to those of equivalent specimens with T300 carbon fibre (CF) which is widely used in industry. From measurements of longitudinal coefficient of thermal expansion (CTE) of the composites we show that the CTE of CNT fibres is approximately ?1.6 × 10^?6 K^?1, similar to in-plane graphite. The combination of electrical, thermal and mechanical properties of CNT fibre composites demonstrates their potential for multifunctionality.     

Processing and impact behavior of Al/SiCp composites fabricated by the pressureless melt infiltration method

In the current investigation, pressureless melt infiltration was applied to fabricate the Al/SiC composites based on the SiC porous preforms. The process was conducted by introducing the aluminum melt into the SiC preforms at 950 °C under the nitrogen atmosphere, without the aid of pressure. To explore development of melt infiltration, initial preforms were produced with variable SiC fractions (40, 50, and 60 vol.%) using three different SiC powders with the mean particle size of 20, 50, and 90 μm. While the infiltration of aluminum melt into the preforms with 40 vol.% initial SiC volume fraction (SiC particle size of 90 μm) resulted to the composites with final density of 0.94 theoretical density (TD), this value drops down to ～0.9 TD for the composites produced by preforms with the SiC (90 μm) volume fraction of 60 vol.%. On the other hand, composites fabricated by 50 μm SiC powder (SiC volume fraction of 40 vol.%) demonstrated the final density of ～0.91 TD. The impact resistance tests performed on the composites demonstrated an enhancement in the value of impact energy with an increase of SiC powder particle size. Results, additionally, revealed a significant superiority of impact energy for the composites fabricated by a combined melt infiltration and sintering (MIS) procedure compared to those produced by infiltration at 950 and 1350 °C.     

A new method to synthesize high solid content waterborne polyurethanes by strict control of bimodal particle size distribution

A new method named two-step emulsification process was developed to synthesize high solid content waterborne polyurethanes by strict control of the bimodal particle size distribution. In the first step, a series of 40% solid content polyester-based (WPU-1) with low content of hydrophilic group and large particle size were firstly synthesized. In the second step, polyether-based prepolymers (WPU-2 prepolymers) with high content of hydrophilic group were firstly prepared and WPU-1 emulsions were used to emulsify WPU-2 prepolymers to obtain the final emulsions with high solid content (WPU-3). The particle size of WPU-3 present bimodal distribution and the diameter ratio and volume percentage of large particles to small particles in WPU-3 were able to be strictly controlled by this method. The viscosity of WPU-3 with 55% solid content was only 489.1 mPa s⁻¹ when the diameter ratio of large particles to small particles was 9.2 and the volume percentage of large particles was 74%.     

Comparison between barium and strontium-glass composites for sealing SOFCs

The present study demonstrates the feasibility of adding micron-scale Y₂O₃-stabilized ZrO₂ (YSZ) powders to modify the properties of two borate glasses used for sealing electrolyte supported SOFCs. The crystallization of the composite made with a Ba-containing glass was found to be independent of the volume fraction of YSZ, as opposed to the situation for Sr-glass composites where the crystallization temperature decreased with the volume fraction of YSZ. The variation of the flow properties of both glass composites was measured using a wettability test, and an increase of the contact angle was measured when the volume fraction of additives was increased. Examining the microstructure showed that initially the Ba-containing glass reacted with YSZ to form a BaZrO₃ compound. Long time exposure at 800 °C caused a large reduction of the coefficient of thermal expansion (CTE), which is explained by increased formation of BaZrO₃ and further change in glass composition. On the other hand, the reaction involving the Sr-containing glass with the YSZ additive shows the initial formation of calcium zirconate (Ca is an ingredient in both glasses) followed by appearance of strontium zirconate with further heating. For this Sr glass composition, the observed reduction of CTE was associated with the change in composition of the remaining glassy phase since the CTEs of the reaction products are close to the CTE of the YSZ additives.     

Investigation of thermal expansion of 3D-stitched C–SiC composites

Carbon fiber reinforced silicon carbide (C–SiC) composites are promising materials for a severe thermo-erosive environment. 3D-stitched C–SiC composites were fabricated using liquid silicon infiltration. The infiltration was carried out at 1450–1650 °C for 10–120 min in vacuum. Coefficient of thermal expansion (CTE) of the composites was determined in in-plane and through-thickness directions in the temperature range from room temperature to 1050 °C. The in-plane CTE varies in the range (0.5–2) × 10^?6/°C, while that in the through-thickness direction, it varies in the range (1.5–4) × 10^?6/°C. The effect of siliconization conditions is higher in the through-thickness direction than in the in-plane direction. The CTE values are lower than the values reported for chemical vapor impregnation based 3D C–SiC composites. An extensive microstructure study was also carried out to understand the thermal expansion behavior of the composites. It was found out that CTE behavior is closely related to the composition of the composite which in turn depends upon siliconization conditions. The best conditions were 1650 °C and 120 min.     

The influence of nano alumina additions on the coefficient of thermal expansion of a LZS glass–ceramic composition

In this work a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (mol%) glass–ceramic matrix was prepared and milled in order to determine its coefficient of thermal expansion (CTE) and to study how it is influenced by the addition of nanosized Al₂O₃ particles (1–5 vol%) and submicrometric Al₂O₃ particles (5 vol%). Comminution studies from the LZS parent glass frit showed that a powder with an adequate particle size (3.5 µm) is achieved after 120 min of dry milling followed by a second step of 60 h wet milling. The obtained LZS glass–ceramic samples (fired at 900 °C/30 min) showed an average relative density of ∼98% with zirconium silicate and lithium disilicate as main crystalline phases. Prepared composites with 1, 2.5 and 5 vol% of nanosized Al₂O₃ and 5 vol% submicrometric Al₂O₃ showed average relative densities varying from 97% to 94% as the alumina content increased. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 9.5 to 4.4×10⁻⁶ °C⁻¹. Composites with 5% nanosized alumina showed a CTE lower than that of the equivalent formulation with submicrometric alumina.     

The effect of particle size of clay on the viscosity build up property of mixed metal hydroxides (MMH) in the low solid-drilling mud compositions

Mixed metal hydroxide (MMH) is a purely inorganic viscosifier of montmorillonite suspensions, having environmental and thixotropic advantages over conventional viscosifiers. It was observed that the particle size distribution of the clay plays a crucial role in building up viscosity of clay–MMH suspensions. Two drilling clays, their separated < 2 μm fractions and a pharmaceutical grade bentonite were studied. It was found that in a clay with 80% (w/w) particles below the 2 μm level the yield point (YP) rise of 2.5% (w/w) aqueous clay suspension is not stopped even on 0.3% (w/v) MMH treatment, whereas in a clay with less than 30% (w/v) particles below the 2 μm level the YP reaches a constant level of 8 lb/100 ft² after 0.1% MMH (w/v) treatment. When the < 2 μm separated fraction is taken from the latter clay and hydrated it was found that YP rises even above the constancy level. On further homo-ionising this separated fraction to Na form and treating with MMH the YP rise was found still higher. When some carefully separated fraction containing a high amount of particles with less than < 1 μm fraction was treated with MMH the YP rise was even higher. With the decrease of the particle size of the clay there is associated an increase of cation exchange capacity (CEC). Therefore, both CEC and particle size play a vital role in viscosity buildup which is due to the formation of an extended gel-network of clay and LDH particles at the prevailing alkaline pH of the system giving rise to high thixotropy. Finally, to reach a bench mark level of YP ≈ 20 lb/100 ft² with the latter clay one must opt for a base mud concentration of a minimum of 4% (w/v) aqueous system.     

High thermal conductive diamond/Ag–Ti composites fabricated by low-cost cold pressing and vacuum liquid sintering techniques

Diamond/Ag–Ti composites were fabricated by a low-cost liquid sintering technique. The Ti addition can effectively improve wetting and promote penetration in composite pores during liquid sintering. The interface structure of the diamond/Ag–Ti composite was identified as Ag/TiC/Ag–Ti/diamond. A high thermal conductivity of 719 W/mK was obtained for the 50 vol.% diamond/Ag-1 at.% Ti composite. Using a bimodal mixture (60 vol.% 150 μm + 10 vol.% 50 μm diamond/Ag-2 at.% Ti composite), a low coefficient of thermal expansion of 6.3 × 10^− 6/K still with high thermal conductivity of 687 W/mK was achieved. These composites have potential applications for thermal management of high integration electronic devices.     

Sintering of glass matrix composites with small rigid inclusions

We investigated the effect of dispersed crystalline particle volume content Φ on sintering of glass matrix composites (GMC) for low-temperature co-fired ceramics (LTCC) applications. Such composites typically consist of alumo-borosilicate glass and α-Al₂O₃ powders of similar average particle size (D₅₀ ≈ 3 μm). Sintering shrinkage was observed by dilatometry and heating microscopy and was backed up by glass viscosity measurements. Microstructure analysis revealed that α-Al₂O₃ particles do neither show significant dissolution into the liquid phase nor detectable crystallization throughout LTCC firing schedules. Therefore, in this study α-Al₂O₃ particles were treated as small rigid inclusions. It was found that Φ lowers the shrinkage rate of GMC. While the lowering is small for small Φ and at the early stage of densification it progressively increases during sintering, and final shrinkage shifts up to 170 K to higher temperatures for Φ = 0.45. The behaviour observed could be explained assuming that sintering is controlled by the effective viscosity, which progressively increases non-linearly during densification due to the gradually wetting of the surface area of corundum particles. We could demonstrate that Al₂O₃ cluster can cause residual pores and reduce the attainable shrinkage. The reduction of attainable shrinkage is found to depend on Φ³, reaching about 8% at Φ = 0.45.     

Thermo-mechanical behaviors and moisture absorption of silica nanoparticle reinforcement in epoxy resins

An investigation of the thermo-mechanical behavior of silica nanoparticle reinforcement in two epoxy systems consisting of diglycidyl ether of bisphenol F (DGEBF) and cycloaliphatic epoxy resins was conducted. Silica nanoparticles with an average particle size of 20 nm were used. The mechanical and thermal properties, including coefficient of thermal expansion (CTE), modulus (E), thermal stability, fracture toughness (K_IC), and moisture absorption, were measured and compared against theoretical models. It was revealed that the thermal properties of the epoxy resins improved with silica nanoparticles, indicative of a lower CTE due to the much lower CTE of the fillers, and furthermore, DGEBF achieved even lower CTE than the cycloaliphatic system at the same wt.% filler content. Equally as important, the moduli of the epoxy systems were increased by the addition of the fillers due to the large surface contact created by the silica nanoparticles and the much higher modulus of the filler than the bulk polymer. In general, the measured values of CTE and modulus were in good agreement with the theoretical model predictions. With the Kerner and Halpin-Tsai models, however, a slight deviation was observed at high wt.% of fillers. The addition of silica nanoparticles resulted in an undesirable reduction of glass transition temperature (T_g) of approximately 20 °C for the DGEBF system, however, the T_g was found to increase and improve for the cycloaliphatic system with silica nanoparticles by approximately 16 °C. Furthermore, the thermal stability improved with addition of silica nanoparticles where the decomposition temperature (T_d) increased by 10 °C for the DGEBF system and the char yield significantly improved at 600 °C. The moisture absorption was also reduced for both DGEBF and cycloaliphatic epoxies with filler content. Lastly, the highest fracture toughness was achieved with approximately 20 wt.% and 15 wt.% of silica nanoparticles in DGEBF and cycloaliphatic epoxy resins, respectively.     

Thermal conductivity of a graphene oxide–carbon nanotube hybrid/epoxy composite

Thermally conductive graphene oxide (GO)–multi-wall carbon nanotube (MWCNT)/epoxy composite materials were fabricated by epoxy wetting. The polar functionality on the GO surface allowed the permeation of the epoxy resin due to a secondary interaction between them, which allowed the fabrication of a composite containing a high concentration of this hybrid filler. The thermal transport properties of the composites were maximized at 50 wt.% of filler due to fixed pore volume fraction in filtrated GO cake. When the total amount of filler was fixed 50 wt.% while changing the amount of MWCNTs, a maximum thermal conductivity was obtained with the addition of about 0.36 wt.% of MWCNTs in the filler. Measured thermal conductivity was higher than the predicted value based on the by Maxwell–Garnett (M–G) approximation and decreased for MWCNT concentrations above 0.4%. The increased thermal conductivity was due to the formation of 3-D heat conduction paths by the addition of MWCNTs. Too high a MWCNT concentration led to increased phonon scattering, which in turn led to decreased thermal conductivity. The measured storage modulus was higher than that of the solvent mixed composite because of the insufficient interface between the large amount of filler and the epoxy.     

Conductivity and anticorrosion performance of polyaniline/zinc composites: Investigation of zinc particle size and distribution effect

Polyaniline/zinc composites and nanocomposites were prepared using solution mixing method. Zinc (Zn) particles with an average particle size of 60 μm and zinc nanoparticles with an average particle size of 35 nm were used as fillers in polyaniline (PANI) matrix. Films and coatings of PANI/Zn composites and nanocomposites were prepared by the solution casting method. Electrical conductivity and anticorrosion properties of PANI/Zn composite and nanocomposite films and coatings with different zinc loadings were evaluated. According to the results, electrical conductivity and anticorrosion performances of both PANI/Zn composites and nanocomposites were increased by increasing the zinc loading. Also results showed that the PANI/Zn nanocomposite films and coatings have better electrical conductivity and corrosion protection effect on iron coupons compared to that of PANI/Zn composite.     

Micronization of salicylic acid and taxol (paclitaxel) by rapid expansion of supercritical fluids (RESS)

The particle sizes of the pharmaceutical substances are important for their bioavailability. The bioavailability can be improved by reducing the particle size of the drug. In this study, salicylic acid and taxol were micronized by the rapid expansion of supercritical fluids (RESS). Supercritical CO₂ and CO₂ + ethanol mixture were used as solvent. Experiments were carried out to investigate the effect of extraction temperature (318–333 K) and pressure (15–25 MPa), pre-expansion temperature (353–413 K), expansion chamber temperature (273–293 K), spray distance (6–13 cm), co-solvent concentration (ethanol, 1, 2, 3, v/v, %) and nozzle configuration (capillary and orifice nozzle) on the size and morphology of the precipitated salicylic acid particles. For taxol, the effects of extraction pressure (25, 30, 35 MPa) and co-solvent concentration (ethanol, 2, 5, 7, v/v, %) were investigated. The characterization of the particles was determined by scanning electron microscopy (SEM), optical microscopy, and LC–MS analysis.The particle size of the original salicylic acid particles was L/D: 171/29–34/14 μm/μm. Depending upon the different experimental conditions, smaller particles (L/D: 15.73/4.06 μm/μm) were obtained. The particle size of taxol like white crystal powders was reduced from 0.6–17 μm to 0.3–1.7 μm The results showed that the size of the precipitated salicylic acid and taxol particles were smaller than that of original particles and RESS parameters affect the particle size.     

Fabrication and characterization of cordierite-bonded porous SiC ceramics

A reaction bonding technique was used for the preparation of cordierite-bonded porous SiC ceramics in air from α-SiC, α-Al₂O₃ and MgO, using graphite as the pore-forming agent. Graphite was burned out to produce pores and the surface of SiC was oxidized to SiO₂ at high temperature. With further increasing the temperature, SiO₂ reacted with α-Al₂O₃ and MgO to form cordierite. SiC particles were bonded by the cordierite and oxidation-derived SiO₂. The reaction bonding characteristics, phase composition, open porosity, pore size distribution and mechanical strength as well as microstructure of porous SiC ceramics were investigated. The pore size and porosity were strongly dependent, respectively, on graphite particle size and volume fraction. The porous SiC ceramics sintered at 1350 °C for 2 h exhibited excellent combination properties, the flexural strength of 26.0 MPa was achieved at an open porosity of 44.51%.     

Effect of mullite additions on the fracture mode of alumina

This work analyses the effect of mullite additions on the fracture mode of alumina. Mullite is proposed as an alternative to SiC for the second phase particles because the thermal expansion mismatch between alumina and mullite is of the same sign and order as that between alumina and SiC. Three alumina–5 vol.% mullite composites formed by alumina matrices with similar average grain sizes in the micrometric range (≈1 μm) and second phase sub-micrometric (50–350 nm) and nanometric mullite (<50 nm) particles located at grain boundaries and triple points were prepared. The fracture mode of the alumina matrix changed from predominantly intergranular to predominantly transgranular. This change became more significant as the size of the sub-micrometric fraction of mullite particles decreased.     

Mechanical properties and sliding wear behaviour of Al2O3-SiC nanocomposites with 3–20 vol% SiC

Al₂O₃/SiC composites containing different volume fractions (3, 5, 10, 15, and 20 vol%) of SiC particles were produced by conventional mixing of alumina and silicon carbide powders, followed by hot pressing at 1740 °C for 1 h under the pressure of 30 MPa in the atmosphere of Ar. The influence of the volume fraction and size of SiC particles (two different powders with the mean size of SiC particles 40 and 200 nm were used), and final microstructure on mechanical properties and dry sliding wear behaviour in ball-on-disc arrangement were evaluated. The properties of the composites were related to a monolithic Al₂O₃ reference. Microstructure of the composites was significantly affected by the volume fraction of added SiC, with the mean size of alumina matrix grains decreasing with increasing content of SiC particles. The addition of SiC moderately improved the Vickers hardness. Fracture toughness was lower with respect to monolithic Al₂O₃, irrespective of the volume fraction and size of SiC particles. Al₂O₃/SiC nanocomposites conferred significant benefits in terms of wear behaviour under the conditions of mild dry sliding wear. Wear resistance of the alumina reference was poor, especially at the applied load of 50 N. The wear rates of composites markedly decreased with increasing volume fraction of SiC. Wear of the composites was also influenced by the material of counterparts, especially their hardness, with softer counterparts resulting in lower wear rates. All composites wore by a combination of grain pull-out with plastic deformation associated with grooving and small contribution of mechanical wear (micro-fracture). No influence of SiC particle size on wear rate or mechanism of wear was observed in the materials with identical volume fractions of SiC.