The effect of milling time on the sintering kinetics of Si3N4 based nanocomposites

Multi-walled carbon nanotube (MWCNT) reinforced silicon nitride composites have been prepared by hot isostatic pressing at 20 MPa and gas pressure sintering at 2 MPa. To assure a good dispersion of the MWCNTs a highly efficient attritor milling was employed in the preparation process of the powder mixtures. The morphological and micro-structural evolution of the powder particles during the high-energy milling was monitored.We have found that the milling time has a complex influence on the structure and mechanical properties of the resulting nanocomposites through affecting both the dispersion and degradation of the nanoscale filler as well as the phase transformations of the ceramic host.     

Advanced multifunctional graphene aerogel – Poly (methyl methacrylate) composites: Experiments and modeling

In this work, graphene aerogel (GA)–poly (methyl methacrylate) (PMMA) composites are first developed by backfilling PMMA into the pores of the GAs, providing uniform distribution of multi-layer reduced graphene oxide (m-rGO) sheets in the PMMA matrix. Electrical, mechanical and thermal properties of the as-prepared GA–PMMA composites are investigated by two-probe, microindentation and comparative infrared techniques respectively. As graphene loadings increase from 0.67 to 2.50 vol.%, the composites exhibit significant increases in electrical conductivity (0.160–0.859 S/m), microhardness (303.6–462.5 MPa) and thermal conductivity (0.35–0.70 W/m K) from that of pure PMMA as well as graphene–PMMA composites prepared by traditional dispersion methods. Thermal boundary resistance between graphene and PMMA is estimated to be 1.906 × 10⁻⁸ m² K/W by an off-lattice Monte Carlo algorithm that takes into account the complex morphology, size distribution and dispersion of m-rGO sheets.     

Bimodal distribution of filler on viscosity and thermal expansion of glass composites

This paper investigates the bimodal oxide filler system to study the viscous behavior and thermal expansion properties of glass composites. Zinc oxide and cordierite, which are two types of filler, with different average diameters (10 μm and 1 μm, respectively), were considered in a Bi2O3 containing glass with various volume fractions (up to 40 vol%). The experimental results for the composites with the bimodal filler distribution show a reduced viscosity. The viscosity increased from fine particles to coarse particles with an increase in the volume fraction of the composite. Both viscosity and coefficient of thermal expansion (CTE) decreased significantly in the composite with the cordierite filler. The CTE is determined from the volume fraction with respect to particle size and distribution. On the other hand, viscosity is dependent on the particle distribution, particle size, and volume fraction of the composite.     

Physical,mechanical, and biological properties of electrophoretically deposited lithium-doped calcium phosphates

In the present work, the preparation of sintered lithium-doped tricalcium phosphates was studied, along with their physical, mechanical, and biological properties. Calcium phosphates were shaped via the use of electrophoretic deposition (EPD), using colloidally milled dispersions of hydroxyapatite (HAp) particles. The dispersions were stabilised with monochloroacetic acid. Lithium was incorporated into the structure via an addition of lithium chloride, which also served to optimise the deposition process. The dispersions were milled colloidally for periods of 0–48 h. The colloidal milling resulted in two effects: i) disintegration of the commercial HAp powder (10 µm) agglomerates, ii) unimodal distribution of the HAp particles (~ 170 nm). The fine particles of the milled HAp dispersions accelerated the deposition rate, and increased the mass of the deposit. The reduced size of the initial particles, owed to the milling, led to the superior arrangement of the particles during deposition and to reduced porosity after sintering (1050–1250 °C). The HAp decomposed into tricalcium phosphate phases during sintering. At a sintering temperature of 1250 °C, grain growth occurred, which consequently resulted in a slight degradation of the mechanical properties (reduction in hardness and Young's modulus). In contrast, the hardness and Young's modulus increased as the dispersion milling time increased (smaller grain size after sintering); however, the fracture toughness did not change. The results of the biological testing confirmed the bioactivity of the material through the growth of the apatite layer in the simulated body fluid (SBF), and the biodegradation of the prepared materials in the Tris-HCl solution. With regard to the preparation of compact lithium-doped tricalcium phosphates, the best results were obtained in the case of the sample that utilised the dispersion that was milled for 48 h, and was sintered at 1050 °C.     

Effects of PMMA on porous structure of pollucite

The effects of PMMA as a pore-forming reagent and the powder for Cs-deficient pollucite, Cs₉Al_0.9Si_2.1O₆, calcined at 1073 K, on the microstructure of the porous body of Cs_0.9Al_0.9Si_2.1O₆ were investigated. The Cs_0.9Al_0.9Si_2.1O₆ porous bodies were fabricated by sintering the green compacts of the calcined powder and PMMA adding 35 mass% to the calcined powder. When the green compact was heated at 873 K in air for 20 h, pores <1 μm were observed in the porous body, suggesting that the PMMA previously dissolved in acetone was uniformly distributed in the calcined powder by the ball milling. The pore size of the obtained porous structure increased with increasing the size of the aggregated particles and the pore size distribution was significantly related to the size of Al₂O₃ balls and the time for the ball milling for mixing the calcined powders and PMMA.     

Synthesis of dispersed CaCO3 nanoparticles by the ultrafine grinding

A one-step grinding process to obtain CaCO₃ nanoparticles from a micrometer-sized CaCO₃ was studied. A high-speed beads mill was employed to grind the particles, and poly(acrylic acid, sodium salt) was used to disperse the ground particles. The main parameters, which were investigated, were the slurry concentration, the rotor speed, the bead size, and the surfactant concentration. The larger bead size, higher slurry concentration, and faster rotor speed showed higher grinding efficiencies. However, there was severe agglomeration of the ground particles resulting in larger secondary particles as the grinding time increased after the certain point. The dispersion and enhanced grinding of particles were achieved by the surfactant. The particle size distribution of the ground particles had a narrow peak around 190 nm that was measured by the diffraction method. The primary particle size of the ground particles was around 40 nm.     

Multi-layer graphene/copper composites: Preparation using high-ratio differential speed rolling,microstructure and mechanical properties

The possibility of using multi-layer graphene (MLG) particles as reinforcement for enhancing the mechanical properties of Cu matrix composites was explored. The combination of ball milling and high-ratio differential speed rolling (HRDSR) techniques was utilized to fabricate the 0.5 and 1 vol.% MLG/Cu composites. In the HRDSR-processed composites, the nanosized MLG particles with 5–15 nm in diameter were dispersed densely and uniformly in the grain interiors of Cu matrix with a preferred crystallographic relationship of 〈1 1 1〉_Cu//〈0 0 0 1〉_MLG to the matrix. The conventionally rolled composites with the same contents of MLG, however, contained much lower densities of nanosized MLG particles. This result indicates that the large shear strain induced during HRDSR accelerated breaking up of MLGs into nanosizes and enhanced their dispersion in the matrix. The strength improvement through the addition of MLGs was obvious when HRDSR was used, but it was negligible when conventional rolling was used. The strengthening gained through the homogeneous dispersion of high-density nanosized MLG particles in the HRDSR-processed composites was attributed to Orowan strengthening. This finding is different from the HRDSR-processed carbon-nanotube (CNT)/Cu composites studied in our previous work, in which the grain-size reduction through the addition of CNTs was the major contribution to the strengthening effect.     

Rheology and applications of highly filled polymers: A review of current understanding

This paper reviews current knowledge about the rheology and applications of highly concentrated molten polymers, focusing on hard particles with sizes ranging from several 100 s nm to a few microns. Understanding the rheological properties should assist the formulation and processing of such polymeric materials. The main factors affecting the rheological behavior of these composites are discussed, such as size distribution, nature and shape of the particles, interactions, maximum packing fraction and matrix viscosity. The matrix viscosity is a key parameter that has to be optimized to be low enough to enable the material processing and high enough to improve the dispersion. The size polydispersity of the fillers facilitates higher filling levels and decreases the melt mixture viscosity for a given filler content. The different types of interactions (viz. particle-particle, particle-matrix) are described to interpret the phenomena arising during processing better. On the other hand, mixing is of particular importance to reach high-quality dispersion and distribution of the particles in the matrix in order to obtain a homogenous mixture and desirable properties. The mixing methods and tools to characterize the degree of mixing are reviewed. The use of organic dispersants is generally necessary to improve and control the dispersion degree and flow properties. Mathematical models relating the viscosity as a function of the filler content for unimodal and bimodal highly filled suspensions are summarized. Constraints and flow instabilities often lead to non-linear rheological behavior such as wall slip, particle-binder segregation, swelling and surface instabilities phenomena; these are discussed. Finally, the latest applications for highly filled systems (such as solid propellants, flame retardancy, magnetic materials, ceramic materials, batteries, etc.) are presented as a source of inspiration for industrial improvements.     

Powder metallurgy Ti–TiC metal matrix composites prepared by in situ reactive processing of Ti-VGCFs system

Titanium metal matrix composites (TMCs) were fabricated via powder metallurgy (P/M) and hot extrusion. Planetary ball milling (PBM) was employed to disperse 0.4–1.0 wt% multiwall carbon nanotubes (VGCFs) with pure Ti powder. The fragmented VGCFs were found dispersing homogenously on the flaked Ti particles surface after PBMed for 24 h. The powder mixture was consolidated at 1073 K by the spark plasma sintering (SPS) process. Hot extrusion was performed at 1273 K with an extrusion ratio of 37:1. The microstructures and mechanical properties of the extruded Ti-VGCFs composites were investigated to evaluate the reactive processing of Ti-VGCFs system. The extruded Ti-VGCFs composites, with a 1.0 wt% VGCFs additive dispersed by PBM, exhibited an excellent tensile strength of 1182 MPa in 0.2% YS and 1179 MPa in UTS, which demonstrated a 143.6% and 80.7% increase compared to these of the extruded pure Ti, respectively. The strengthening mechanism was investigated and elucidated that the mechanical strength was attributed to the grain refinement and dispersion strengthening of the homogenously dispersed, in situ formed TiC particulates, as well as a solid solution strengthening of the carbon, oxygen and nitrogen elements in the Ti matrix.     

Brazing SiC ceramic using novel B4C reinforced Ag–Cu–Ti composite filler

The development of new composite fillers is crucial for joining ceramics or ceramics to metals because the composite fillers exhibit more advantages than traditional brazing filler metal. In this research, novel B₄C reinforced Ag–Cu–Ti composite filler was developed to braze SiC ceramics. The interfacial microstructure of the joints was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of B₄C addition and brazing temperature on the microstructure evolution and mechanical properties of the joints was analyzed. The results revealed that TiB whisker and TiC particles were simultaneously synthesized in the Ag-based solid solution and Cu-based solid solution due to the addition of B₄C particles. As the brazing temperature increased, the thickness of Ti₃SiC₂+Ti₅Si₃ layers adjacent to SiC ceramic increased. Desirable microstructure similar to the metal matrix reinforced by TiB whisker and TiC particles could be obtained at brazing temperature of 950 °C. The maximum bending strength of 140 MPa was reached when the joints brazed at 950 °C for 10 min, which was 48 MPa (~52%) higher than that of the joints brazed using Ag–Cu–Ti filler.     

Synthesis of nanodiamond-reinforced aluminum metal matrix composites using cold-spray deposition

A dense nanodiamond–aluminum (ND–Al) composite coating was successfully produced by low pressure cold spray (CS) deposition of ball-milled powders containing 10 wt% ND. High-energy ball milling is a feasible means for the synthesis of composite feedstock powders as it provides excellent control over particle size distribution, crystal size, and the dispersion of ND agglomerates. The resulting CS coatings were characterized with respect to deposition efficiency, particle velocity and mechanical properties. It was found that the CS deposition produced dense, ND–Al composite coatings with increases in both hardness and elastic modulus as compared to the feedstock powders. The coating hardness of the 0.5 h-milled ND–Al composite that has the highest DE (14.2%) in ND–Al composites is 3.02 GPa, an 175% increase over the pristine as-received Al (1.10 GPa). The highest elastic modulus of the composite coatings is 98.3 GPa, a 51.5% increase over the as-received Al powder.     

Synthesis of nanodiamond-reinforced aluminum metal composite powders and coatings using high-energy ball milling and cold spray

Nanodiamond-reinforced aluminum metal matrix composites (ND–Al MMC) powders were synthesized by means of high energy ball milling. We present a systematic study of the effect of various milling conditions on the structure and properties of the resulting MMC powders. The described method can be used to control important powder characteristics, including particle size and shape, Al crystal size and residual strain, and structural integrity and dispersion of the nanoparticle inclusions, a crucial requirement for subsequent powder consolidation. Raman spectroscopy was utilized for the first time to directly verify the structural integrity and the dispersion of ND in the Al matrix. For low ball-to-powder ratios (BPR), average particle size and size range of the ND–Al composite powders were found to decrease during milling, while the hardness increases. A BPR of 10:1, a milling time of 10 h, and a ND content of 10 wt.% were most effective in obtaining small powder particle sizes, small Al crystal sizes, and improved mechanical properties reaching a hardness of 3.46 GPa, a 210% increase over the pristine, untreated Al powder (1.10 GPa). Finally, we demonstrate that the as-produced composite powders are well-suited for low-temperature consolidation processing by fabricating the first cold-sprayed ND–Al MMC coating.     

Flame retardancy and thermal stability of ethylene-vinyl acetate copolymer nanocomposites with alumina trihydrate and montmorillonite

The development of fire retardant for wire and cable sheathing materials has oriented toward low smoke and halogen-free flame retardant technology to achieve better safety for electrical equipment and devices and to satisfy standards. However, many polymer flame resistance materials require a very high proportion of metal hydrate filler within the polymer matrix (60 wt%) to achieve a suitable level of flame resistance, which may lead to inflexibility, poor mechanical properties and problems during compounding and processing. In this study, the alumina trihydrate (ATH) was added to montmorillonite (MMT) as the halogen-free flame retardant of ethylene-vinyl acetate (EVA) copolymer, with various ratios of EVA/ATH/MMT. The prepared nanocomposites were characterized through various techniques of XRD, tensile test, DSC analysis, TGA, LOI evaluation, and FE-SEM to explore the effects of organic modified clay (OMMT) and the layer distance on the mechanical, thermal, and flame resistance properties. In the XRD examinations, the layer-distance of MMT increased from 1.27 to 1.96 nm when polymer was added to the octadecylamine modified MMT. The best tensile strength was obtained at 3 wt% MMT. In addition, the halogen-free flame resistance grade of EVA containing 3 wt% OMMT and 47 wt% ATH revealed the best elongation and fire resistance (LOI = 28). The tensile and flame resistance properties of the nanocomposites were also significantly improved.     

Properties of polypropylene/aluminum trihydroxide composites containing nanosized organoclay

Montmorillonite is a promising substitute for aluminum trihydroxide in flame‐retardant polypropylene/aluminum trihydroxide (PP/ATH) composites. Study was made of the partial substitution of organoclay for ATH in PP/ATH composites. The total concentration of filler was kept at 30 wt%. The composites were compatibilized with two types of compatibilizer: commercial maleic anhydride functionalized polypropylene (PP‐g‐MA) and hydroxyl‐functionalized polypropylene (PP‐co‐OH) prepared with metallocene catalyst. The effect of compatibilization on the morphology was studied by the transmission electron microscopy and the scanning electron microscopy. Mechanical properties were characterized by tensile and impact measurements, and flammability properties with a cone calorimeter. Addition of compatibilizer and stearic acid (SA) treatment of the ATH particles contributed to the dispersion of the fillers. Both compatibilizers produced organoclay with exfoliated structure and improved adhesion between the fillers and the matrix. Toughness improved and decomposition and flammability were reduced. POLYM. ENG. SCI. 45:1568–1575, 2005. © 2005 Society of Plastics Engineers     

Improved strength and toughness of polyketone composites using extremely small amount of polyamide 6 grafted graphene oxides

Polyketone (PK) composites were prepared by a solution casting method using 1,1,1,3,3,3-hexafluoro-2-propanol as a solvent and polyamide 6 grafted graphene oxides (PA 6-GOs) as filler materials. PA 6-GOs were obtained by in situ polymerization of ε-caprolactam using GOs having different amounts of oxygen functional groups. The PK composites containing only an extremely small amount of the PA 6-GOs (0.01 wt%) showed much improved mechanical properties compared to PK. This could be ascribed to the homogeneous dispersion of the graphene-based filler materials in the polymer and specific interactions such as dipole–dipole interactions and/or the hydrogen bonds between the fillers and the polymer matrix. For example, when 0.01 wt% of PA 6-GO having less oxygen functional groups was used as a filler for the composite, the tensile strength, Young’s modulus, and elongation at break of the composite increased by 35%, 26%, and 76%, respectively. When 0.01 wt% of PA 6-GO having larger content of oxygen functional groups and PA 6 was used, Young’s modulus decreased, while the tensile strength increased by 37%, and the elongation at break increased tremendously by 100 times, indicating that very tough polymeric materials could be prepared using a very small amount of the graphene-based fillers.     

Silicon oxycarbide-based composites produced from pyrolysis of polysiloxanes with active Ti filler

Phenyl (PPS) and methyl (PMS) containing polysiloxanes were pyrolyzed at elevated temperatures (900–1500 °C) under argon atmosphere to investigate the phase developments within the polymers. It was found that pyrolysis of the polymers under inert atmosphere up to 1300 °C leads to amorphous silicon oxycarbide (SiO_xC_y) ceramics. Conversions at higher temperatures results in the transformations into the crystalline β-SiC phases. Ceramic matrix composites (CMCs) were developed based on the active filler controlled pyrolysis (AFCOP) of polysiloxanes with active Ti filler additions. CMC monoliths were prepared with 60–80 wt.% of active Ti particulates blended into polymer precursors. Green bodies of the composites were made by warm pressing under 15 MPa pressure and ceramics were obtained by pyrolysis at elevated temperatures between 900 and 1500 °C under argon atmosphere. The results showed that due to the incorporation of active Ti fillers, formation of crystalline phases such as TiC, TiSi, and TiO occured within the amorphous matrix due to the reactions between the Ti and the polymer decomposition products. The microstructural and mechanical characterization results of the composites are presented within the paper.     

Preparation of presintered zirconia blocks for dental restorations through colloidal dispersion and cold isostatic pressing

This work proposes an effective method for dispersion of zirconia suspension for dental block preparation and optimizes the cold isostatic pressing (CIP) pressure to improve the densification of slip-casted zirconia blocks. Two batches of 44 wt% zirconia suspension were prepared using distilled water in a pH 2 medium containing 0.5 wt% polyethyleneimine as dispersant. The first batch was sonicated for different durations (from 5 min to 30 min), and the second batch was dispersed through ball milling at rotational speeds of 200, 300, and 400 rpm for 60, 90, and 120 min. All suspensions were subjected to sedimentation test and particle size measurement. Results revealed that the optimum ultrasonication duration was 10 min, which yielded the smallest particle size of 133 nm. Ball milling at 300 rpm for 120 min achieved the maximum dispersion of particles, with an average size of 75 nm. Under the optimum conditions of ultrasonication duration, ball milling duration, and ball milling speed, the particle size decreased to 48 nm, which is close to the primary particle size. These dispersion techniques and parameters were selected for preparing a suspension to be consolidated into blocks through slip casting and were enhanced through CIP at pressure ranging from 100 MPa to 300 MPa. CIP compaction at 250 MPa significantly increased the shrinkage percentage of green zirconia blocks, with pore radius decreased to 18 nm. The density of zirconia pressed at 250 MPa and presintered at a low temperature of 950 °C was 59% of the theoretical density and was higher than that of commercial presintered blocks. Thus, CIP should be conducted under a compaction pressure of 250 MPa to produce dense and homogeneous zirconia blocks.     

Brazing ZrO2 ceramic and TC4 alloy by novel WB reinforced Ag-Cu composite filler: Microstructure and properties

Residual stresses in ceramic-metal joints is the important factor for their reliable implementation in cutting-edge industries. Composite fillers is reported to be a promising approach to reduce the residual stresses. Until now, few experimental researches on the brazing of ZrO₂ ceramic and TC4 alloy using composite fillers have been reported. In this study, to release the residual stresses and improve the joints strength, novel WB reinforced Ag-Cu composite filler was fabricated to braze ZrO₂ ceramic and TC4 alloy. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) were applied for the analysis of microstructure and phases structure in the joints. The TiB whiskers and W particles were in situ synthesized via the reaction between active Ti and WB particles, and randomly distributed in the brazing seam. The effect of brazing temperature and WB content on interfacial microstructure and mechanical strength in the brazed joints were investigated. When brazed at 870 °C for 10 min, favorable microstructure reinforced by TiB whiskers and W particles in the brazing seam was achieved with 7.5 wt% WB addition in composite filler. The maximum average shear strength of the joints was 83.2 MPa, which about 59.4% increase over the joints without WB addition.     

The effect of surface chemistry of graphene on rheological and electrical properties of polymethylmethacrylate composites

Graphene sheets with different oxygen contents were prepared to functionalize the electrically insulating polymethylmethacrylate (PMMA). The influences of surface chemistry of graphene on rheological, electrical and electromagnetic interference (EMI) shielding properties of its PMMA composites were investigated. The appearance of frequency-independent storage modulus at low frequency suggests a solid-like viscoelastic behavior and the formation of an interconnected network of graphene in the matrix. Due to the favorable interfacial interactions arising from polarity matching, the graphene with a C/O ratio of 13.2 (graphene-13.2) shows a better dispersion in PMMA than those with lower C/O ratios, and thus its PMMA composites exhibit lower rheological and electrical percolation thresholds. The EMI shielding properties of the graphene/PMMA composites exhibit similar dependence on the oxygen content of graphene. A high EMI shielding effectiveness of ～30 dB was obtained for the PMMA composite with 4.2 vol.% of graphene-13.2 with microwave absorption as the dominant EMI shielding mechanism.     

Solubility and diffusivity of CO2 in polypropylene/micro-calcium carbonate composites

This work is aimed at studying the effects of the fillers and interface bonding condition between the fillers and polymer matrix on the solubility and diffusivity of CO₂ in polypropylene (PP)/Micro-calcium carbonate (MicroCaCO₃) composites. The solubility of CO₂ in PP and its composites containing 5% and 10% MicroCaCO₃ was determined precisely by using magnetic suspension balance (MSB) combined with experimental swelling correction at 200 and 220 °C and CO₂ pressures up to 22 MPa. It was found that the solubility of CO₂ in the PP/MicroCaCO₃ composites without the interface compatibilizer increased with increasing the filler content, while the CO₂ solubility remained almost unchanged in PP composites with compatibilizer. The Henry's law and a modified Henry's law were used to well correlate the solubility of CO₂ in the PP composites with and without the interface compatibilizer, respectively. The diffusion coefficient of CO₂ in the PP composites was found to decrease with increasing the filler content. The mutual diffusion coefficients of CO₂ in the PP composites can be correlated within an average relative deviation of 10% by the free volume model proposed by Kulkarni and Stern with a parameter accounting for the barrier effect of the filler.