Structure and properties of mixtures based on long chain polyacrylate and 1-alcohol composites

A series of phase change materials (PCMs) based on long chain polyacrylate and 1-alcohol, i.e., poly (stearyl methacrylate) and 1-tetradecanol (PSMA/C14OH) were prepared through the solution-mixing method. Thermal energy storage capacity, thermal stability and morphology of PSMA/C14OH PCMs were characterized by Fourier transform infrared spectroscopy (FTIR), polarized optical microscopy (POM), field emission scanning electron microscopy (FE-SEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). DSC results demonstrated that the heat of fusion of PSMA/C14OH PCMs increased from 85.9 to 172.3 J g⁻¹ with the weight fraction of C14OH increasing from 20 to 80 wt%. And, the thermal stability also enhanced with PSMA weight. The spherulite (ca. 250 μm) in PSMA/C14OH composites containing 60 wt% C14OH proved the compatibility between PSMA and C14OH, indicating the cocrystallization behavior of alkyl side groups appeared. The cocrystallization behavior contributes the enhanced thermal stability of PSMA/C14OH PCMs, and it is suitable as the thermal energy storage materials in the future.     

Reaction pathways,activation energies and mechanical properties of hybrid composites synthesized in-situ from Al–TiO2–C powder mixtures

In-situ aluminum matrix composites were fabricated from Al–TiO₂–graphitic C powder mixtures using exothermic dispersion method. The effects of C/TiO₂ molar ratio on the reaction processes, activation energies and mechanical properties of the resulting materials were investigated. When the C/TiO₂ molar ratio is 0, Al reacts with TiO₂ to produce fine α-Al₂O₃ particles and Ti, which then reacts with Al to form large rod-like Al₃Ti phase. By adding graphite C into the Al–TiO₂ system, the activation energy of the first reactive step increases; in addition, the resultant Ti preferentially reacts with C to form hard TiC particles. When the C/TiO₂ molar ratio increases to 1.0, the Al₃Ti phase disappears and the reinforcements consist of nano-sized α-Al₂O₃ and TiC phases. The tensile strength of the composites increases from 239.2 MPa to 351.8 MPa and the elongation increases from 4.1% to 5.6%, suggesting a marked increase in damage tolerance (i.e., toughness).     

Reaction mechanism and mechanical properties of an aluminum-based composite fabricated in-situ from Al–SiO2 system

An (α-Al₂O₃ + Si)/Al composite was fabricated by an in situ process called exothermic dispersive synthesis from a powder blend of Al and SiO₂. The synthesis mechanism and resulting microstructure of the composite were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-ray energy dispersive spectroscopy (EDS). It was found that when the reinforcement volume fraction was 30%, Al reacted with SiO₂ at a temperature of around 950 K to form α-Al₂O₃ and Si, and the corresponding apparent activation energy was 235 kJ mol⁻¹. The apparent activation energy of the reaction decreases with the increase of the reinforcement volume fraction. On the other hand, with increasing heating rate, the reaction time was reduced and the characteristic exothermic peak shifted to a higher temperature. The tensile strength and elongation of the composite having the reinforcement volume fraction of 30% were 165 MPa and 3.95%, respectively. On the tensile fracture surfaces, there were many fine dimples in addition to some fractured blocks. The α-Al₂O₃ particles were located in the center of the fine dimples. The Si crystal blocks fractured along certain cleavage plane in a brittle manner and are a limiting factor to the tensile property of the newly-developed composite. Therefore, the elimination or replacement of the Si crystals by a tougher phase is an important consideration for future work.     

In situ NiTi/Nb(Ti) composite

This paper reports on the creation of a series of in situ NiTi/Nb(Ti) composites with controllable transformation temperatures based on the pseudo-binary hypereutectic transformation of NiTi–Nb system. The composite constituent morphology was controlled by forging and rolling. It is found that the thickness of the NiTi lamella in the composite reached micron level after the hot-forging and cold-rolling. The NiTi/Nb(Ti) composite exhibited high damping capacity as well as high yield strength.     

Nonisothermal crystallization behavior and mechanical properties of PEEK/SCF/nano-SiO2 composites

Nonisothermal crystallization of hybrid PEEK composites reinforced with short carbon fibers (SCF) and nano-SiO₂ (1, 1.5 and 2 wt%) was investigated using DSC. Composites were fabricated by melt-mixing process at 400 °C. The Size of the nanoparticles was 13 nm. Samples were cooled from 410 °C to 25 °C with cooling rates of 10, 30, 50 and 70 °C min⁻¹. The onset, peak and end crystallization temperatures were investigated as well as absolute crystallization percentage and crystallization time. Avrami, Ozawa and Ozawa–Avrami equations were fitted to the data in order to investigate the crystallization kinetics. Mechanical behaviors of the composites were examined using nanoindentation and nanoscratching. DSC results revealed that absolute crystallization percentage increases in PEEK/SCF/1%SiO₂ and PEEK/SCF/1.5%SiO₂ samples compared to PEEK/SCF, however it decreases by adding more nano-SiO₂. Ozawa–Avrami is proved to be the best model for describing crystallization behavior of the composites while Avrami equation was suitable for describing a part of the crystallization process. The Avrami and Ozawa–Avrami constants were calculated. Besides, adding SCFs and nano-SiO₂ into PEEK results in a significant decrease in plasticity index, while increases the resistance to plastic deformation of the composite.     

Hydration dynamics of collagen/PVA composites: Thermoporometric and impedance analysis

Porous scaffolds like collagen/PVA (polyvinyl alcohol) composites have potential applications in the field of biomedical engineering. The pore properties and electrical behavior of collagen/PVA composite system were investigated by thermoporometry technique and electrochemical impedance analysis. The porous composites were crosslinked by less cytotoxic genipin due to the versatility in the crosslinking reactivity between the amino groups. Different physicochemical properties like rheological behavior, thermal stability of the protein and morphological changes of the composites were investigated as a function of PVA concentration by viscosity profile, temperature dependant circular dichroic spectroscopic studies, scanning electron microscopy. Bound water constrained within the pores of collagen/PVA composites seems to provide signatures for changes induced by amount of additives on the pore diameter and distribution in composite molecules. Impedance measurements of the composites in the frequency range of 10⁻² to 10⁵ Hz reveal that concentration of the additive and crosslinking significantly influence the permittivity of the composites. The tunable physicochemical properties help to gain insight for regulating cellular events for tissue and organ regeneration.     

Synthesis,characterization, thermal and dielectric properties of pure and cadmium doped calcium hydrogen phosphate

The synthesis of pure and cadmium doped calcium hydrogen phosphate as single crystal has been accomplished by a room temperature solution growth technique viz., silica gel technique. Silica gel obtained from sodium metasilicate with a strongly acidic cation exchanger in the H-form, was used for crystal growth experiments. The nature of the grown material was established by powder X-ray diffraction (PXRD) studies, whereas from single crystal X-ray diffraction (SXRD) the crystal system comes out to be monoclinic. The stoichiometry of the grown composition was established by energy dispersive X-ray analysis (EDAX). Fourier transform infrared spectroscopic (FTIR) studies signifies the presence of phosphate (PO₄)²⁻ group and water of crystallization. Thermo gravimetrical analysis and investigations of dielectric properties were undertaken to study the thermal stability, dielectric constant and transition temperature of the grown material. Dielectric studies suggest that there is a shift in the value of transition temperature (T_c) thereby indicating relaxor behaviour of the material.     

Evaluation of microstructure and mechanical properties of Al/Al2O3/SiC hybrid composite fabricated by accumulative roll bonding process

In this investigation, a new kind of metal matrix composites with a matrix of pure aluminum and hybrid reinforcement of Al₂O₃ and SiC particles was fabricated for the first time by anodizing followed by eight cycles accumulative roll bonding (ARB). The resulting microstructures and the corresponding mechanical properties of composites within different stages of ARB process were studied. It was found that with increasing the ARB cycles, alumina layers were fractured, resulting in homogenous distribution of Al₂O₃ particles in the aluminum matrix. Also, the distribution of SiC particles was improved and the porosity between particles and the matrix was decreased. It was observed that the tensile strength of composites improved by increasing the ARB passes, i.e. the tensile strength of the Al/1.6 vol.% Al₂O₃/1 vol.% SiC composite was measured to be about 3.1 times higher than as-received material. In addition, tensile strength of composites decreased by increasing volume fraction of SiC particles to more than 1 vol.%. Scanning electron microscopy (SEM) observation of fractured surfaces showed that the failure mechanism of broken hybrid composite was shear ductile rupture.     

Acrylonitrile-capped poly(propyleneimine) dendrimer curing agent for epoxy resins: Model-free isoconversional curing kinetics,thermal decomposition and mechanical properties

Acrylonitrile-modified aliphatic amine adducts are often used as curing agents for room-temperature epoxy formulations (coatings, adhesives, sealants, castings, etc.), yet the curing reaction and properties of resultant epoxy systems still remain less fundamentally understood. Herein we systematically investigate our newly-developed acrylonitrile-modified multifunctional polyamine curing agent for bisphenol A epoxy resin (DGEBA): an acrylonitrile-capped poly(propyleneimine) dendrimer (PAN4). The impact of the molecular structure of PAN4 and a controlled poly(propyleneimine) dendrimer (1.0GPPI) on the curing reactivity, reaction mechanisms, thermal stability, viscoelastic response and mechanical properties of the epoxy systems are highlighted. Differential scanning calorimetry (DSC) confirms DGEBA/PAN4 shows markedly lower reactivity and reaction exotherm than DGEBA/1.0GPPI, and the model-free isoconversional kinetic analysis reveals that DGEBA/PAN4 has the generally lower reaction activation energy. To be quantitative, the progress of the isothermal cure is predicted from the dynamic cure by using the Vyazovkin equation. The isothermal kinetic prediction shows that DGEBA/PAN4 requires about 10 times longer time to achieve the same conversion than DGEBA/1.0GPPI, which agrees with the experimentally observed much longer gel time of DGEBA/PAN4. Subsequently, dynamic mechanical analysis shows that PAN4 results in the cured epoxy network with the lower β- and glass-relaxation temperatures, crosslink density, relaxation activation energy, enthalpy, entropy, but the higher damping near room temperature than 1.0GPPI. Finally, thermogravimetric analysis (TGA) demonstrates cured DGEBA/PAN4 is thermally stable up to 200 °C, and mechanical property tests substantiate that PAN4 endows the cured epoxy with much higher impact and adhesion strengths than 1.0GPPI. Our data can provide a deeper insight into acrylonitrile-modified aliphatic amine curing agents from the two good model compounds (PAN4 and 1.0GPPI).     

Effect of submicron size SiC particles on microstructure and mechanical properties of AZ31B magnesium matrix composites

The magnesium matrix composites reinforced with three volume fractions (3, 5 and 10 vol.%) of submicron-SiC particles (∼0.5 μm) were fabricated by semisolid stirring assisted ultrasonic vibration method. With increasing the volume fraction of the submicron SiC particles (SiCp), the grain size of matrix in the SiCp/AZ31B composites was gradually decreased. Most of the submicron SiC particles exhibited homogeneous distribution in the SiCp/AZ31B composites. The ultimate tensile strength and yield strength of the 10 vol.% SiCp/AZ31B composites were simultaneously improved. The study of interface between the submicron SiCp and the matrix in the SiCp/AZ31B composite suggested that submicron SiCp bonded well with the matrix without interfacial activity.     

Nano-AlN particle reinforced Mg composites: microstructural and mechanical properties

In this study, nano-AlN particles were introduced into pure Mg matrix through the powder metallurgy technique incorporating microwave assisted two-directional sintering followed by hot extrusion. The effect of varying volume fraction of nano-AlN addition on the microstructural and mechanical properties of pure Mg was investigated. Microstructural characterisation revealed marginal grain refinement due to the fairly uniform distribution of AlN nano-particulates. X-ray diffraction results indicated basal texture weakening in Mg/0·2AlN composite. Tensile property measurements revealed an overall increase in strength properties and ductility. Among the developed composites, Mg/0·8AlN displayed superior strength (～30% improvement) and Mg/0·2AlN showed enhanced ductility (～80% enhancement). Under compressive loading, the developed Mg/AlN nanocomposite formulations exhibited improved strength properties without significant effect on compressibility.     

Effect of the TiO2 nanoparticle size on the decomposition behaviors in aluminum matrix composites

The decomposition behaviors and the effect of particle size on the kinetic rate are studied for Al–3 vol.% titanium dioxide (TiO₂) composites by using three different types of TiO₂ particles (15, 50, and 300 nm). Thermal analysis shows that the reaction is stepwise with the first reaction starting before the melting temperature of Al. Since the high chemical potential of nanoparticles enhances reactivity, the TiO, Al₃Ti, and α-Al₂O₃ phases are found to be formed during the first reaction regardless of particle size. Based on observations of microstructure, the formation mechanism of Al₃Ti and α-Al₂O₃ is understood to be solution precipitation. Non-isothermal kinetic analysis reveals that the reaction mechanism is closely related to the three-dimensional continuous nucleation and the growth limited by diffusion. Particle size is found to be having considerable effect on the kinetic rate. As the particle size decreases, the rate constant increases, while the pre-exponential factor and the activation energy decreases. A non-linear relationship between the rate constant and the reciprocal of the size is found and evaluated.     

Influence of in situ formed ZrB2 particles on microstructure and mechanical properties of AA6061 metal matrix composites

Particulate reinforced metal matrix composites (PMMCs) have gained considerable amount of research emphasis and attention in the present era. Research is being carried out across the globe to produce new combination of PMMCs. PMMCs are prepared by adding a variety of ceramic particles with monolithic alloys using several techniques. An attempt has been made to produce aluminium metal matrix composites reinforced with zirconium boride (ZrB₂) particles by the in situ reaction of K₂ZrF₆ and KBF₄ salts with molten aluminium. The influence of in situ formed ZrB₂ particles on the microstructure and mechanical properties of AA6061 alloy was studied in this work. The in situ formed ZrB₂ particles significantly refined the microstructure and enhanced the mechanical properties of AA6061 alloy. The weight percentage of ZrB₂ was varied from 0 to 10 in steps of 2.5. Improvement of hardness, ultimate tensile strength and wear resistance of AA6061 alloy was observed with the increase in ZrB₂ content.     

Crystallization kinetics and densification of YAG nanoparticles from various chelating agents

Yttrium aluminium garnet (YAG, Y₃Al₅O₁₂) nanoparticles were prepared using sonochemical sol-gel method with three different chelating agents and the effect of crystallization kinetics was investigated with differential scanning calorimetry-thermogravimetry (DSC-TG). The activation energy values of crystallization for the as-synthesized YAG nanoparticles using citric acid (CA), glycine (G) or a mixture of citric acid-glycine (CA-G), as chelating agents were found to be 160.5, 142.2 and 140.4 kJ mol⁻¹ and the corresponding Avarami constants were 2.2, 2.1 and 1.9, respectively. Samples produced with the mixed chelating agent under sonification, could be crystallized to single phase YAG nanoparticles (10-65 nm) after annealing at 1100 °C. Pellets made from the annealed YAG particles could be sintered to a relative density greater than 99% at 1500 °C with a grain size of 4.5 μm, made up of secondary particles formed from primary nano-crystals within the grains. Grain size and relative density increased with different chelating agents from CA to G and CA-G in the increasing order when YAG samples were sintered. Grain growth and densification occurred at a relatively low temperature of 1500 °C as compared to over 1800 °C in solid-state reactions.     

Influence of TiBw volume fraction on microstructure and high-temperature properties of in situ TiBw/Ti6Al4V composites with TiBw columnar reinforced structure fabricated by pre-sintering and canned extrusion

Tailoring TiBw volume fraction was utilized in TiBw/Ti6Al4V composites to pursue performance optimization for potential high-temperature applications. Detailed investigations focused on the influence of TiBw volume fraction on microstructure and its correlations with mechanical properties mainly at high temperatures ranging from 500 °C to 700 °C. Highly aligned TiB whiskers along extrusion direction caused the formation of TiBw columnar reinforced structure, which was composed of ductile TiBw-poor interior regions and hardened TiBw-rich boundaries. Refined prior β phases arising from dynamic recrystallization led to the size reduction in α colonies. This tendency commonly accompanied with the formation of equiaxed α phase was evidently enhanced by the restriction of increasing TiB whiskers at the boundaries. Much better strengthening of TiB whiskers was achieved on the premise of good interfacial bonding with Ti matrix at high temperatures, and the strengthening depended linearly on their volume fractions, with strength increments of about 44.3 MPa/vol.% at 500 °C, and 27.5 MPa/vol.% at 600 °C, yet merely 6.3 MPa/vol.% at 700 °C. Attributed to the softening of Ti matrix and the crack retardation of ductile TiBw-poor regions by raising temperature, enhanced ductility was negatively correlated with TiBw volume fraction below 600 °C, but positively at 700 °C under the possible grain boundary sliding inspired by ample equiaxed α phase.     

The properties of polyurethane hybrid materials based on castor oil

The goal of this work was to investigate the properties of environmentally friendly, castor oil based polyurethane hybrid materials with titanium(IV) oxide nanoparticles, as a filler, and different types of diisocyanate (toluene diisocyanate and isophorone diisocyanate). In the sample synthesis, different ratios of the reactive groups (NCO/OH), r, were used (1, 1.15 and 0.92). In the composite preparation, only toluene diisocyanate was used, and the filler particles were premixed in a glass vessel with the castor oil polyol before the reaction with diisocyanate. For all the composite samples, the r value was 1. Polyurethane formation was confirmed by ATR-FT-IR by detecting the urethane band at 1515 cm⁻¹. It was determined that the hydroxyl groups had reacted because the broad band corresponding to the OH groups (3400 cm⁻¹) was not detected or detected at a reduced intensity depending on the r value. As was expected, the presence of the unreacted NCO groups was detected only for samples with r > 1 (band at 2300 cm⁻¹, which corresponds to the existence of these groups). The dynamic mechanical measurements were performed at a temperature range from −50 °C to 100 °C at different frequencies. For investigation of reinforcement effect of filler on polymer matrix, tensile testing was applied. The glass transition temperature, T_g, was determined by DSC measurement. It was estimated that the T_g of the samples decreased as the nanofiller content increased due to the changes in the segmental mobility influenced by the interaction between the nanoparticles and polymer chains.     

Coupling of conductivity to the relaxation process in polymer electrolytes

Conductivity relaxation using modulus formalism has been used to explore the coupling of ionic conductivity to dielectric relaxation in polymer electrolyte based on polyethylene oxide complexed with various content of LiAsF₆. The temperature dependence of conductivity followed the VTF behavior suggesting close correlation between conductivity and the segmental relaxation process in polymer electrolytes. The coupling of conductivity to the segmental process has been discussed in terms of coupling index. For all compositions studied, the coupling index was within the range of 1–11 in the temperature range of investigation, which was in agreement with the coupled systems.     

Microstructure and mechanical properties of in situ TiB reinforced titanium matrix composites based on Ti–FeMo–B prepared by spark plasma sintering

The in situ synthesized TiB reinforced titanium matrix composites have been prepared by spark plasma sintering at 800–1200 °C under 20 MPa for 5 min. The effects of sintering temperature and reinforcement volume fraction on flexural strength, Young’s modulus and fracture toughness of the composites are investigated. The titanium matrix consists of -Ti and β-Ti phases, and the volume fraction of β-Ti increases with increasing sintering temperatures. The in situ synthesized TiB reinforcements are distributed randomly and uniformly in matrix. The transverse section of TiB has a hexagonal shape aligned along [0 1 0] direction, and the crystallographic planes of the TiB needles are always of the type . The 10 vol% TiB reinforced composite sintered at 1000 °C exhibits excellent mechanical properties. The flexural strength, Young’s modulus and fracture toughness of this composite are 1560 MPa, 137 GPa and 8.64 MPa · m^1/2, respectively.     

Study of crystallization kinetics and structural relaxation behavior in phase separated Ag33Ge17Se50 glassy alloys

We report on the crystallization processes and structure (crystal phases) of Ag₃₃Ge₁₇Se₅₀ glassy alloy using differential scanning calorimetry and x-ray diffraction techniques, respectively. The devitrification that gives rise to the first exothermic peak results in the crystallization of Ag₂Se and Ag₈GeSe₆ phases, while the growth of GeSe₂ accompanied by the transformation of Ag₈GeSe₆ to Ag₂Se phase occurs during the second crystallization process. Different theoretical models are used to elucidate various kinetic parameters for the crystallization transformation process in this phase separated system. With annealing below the glass transition temperature, an inverse behavior between the variation of the optical gap and the band tailing parameter is observed for the thermally evaporated films. These results are explained as the mixing of different clusters/species in the amorphous state and/or changes caused by structural relaxation of the glassy network for the thermally evaporated films.     

Fabrication and characterization of biodegradable composites based on microfibrillated cellulose and polyvinyl alcohol

Microfibrillated cellulose (MFC) based thin membrane-like fully biodegradable composites were produced by blending MFC suspension with polyvinyl alcohol (PVA). Desired MFC content in the composites could be easily obtained by varying the PVA solution concentration. Chemical crosslinking of PVA was carried out using glyoxal to increase the mechanical and thermal properties of the composites as well as to make the PVA partially water-insoluble. Examination of composite surfaces and fracture topographies indicated that the MFC fibrils were well bonded to PVA and uniformly distributed. Infrared spectroscopy showed that acetal linkages could be formed in the MFC–PVA composites by a glyoxal crosslinking reaction. Sol–gel and swelling results indicated that crosslinking reaction made PVA partially insoluble and reduced its swelling ability. The MFC–PVA composites had excellent tensile properties which were further enhanced by crosslinking. Thermogravimetric analysis (TGA) showed higher thermal stability for MFC–PVA composites compared to PVA. The crosslinked MFC–PVA composites showed even higher thermal stability. Differential scanning calorimetry (DSC) indicated that crosslinking increased the glass transition temperature and reduced melting temperature and crystallinity of PVA in MFC–PVA composites. Results also indicated that nano- and micro-fibrils in MFC inhibit the crystallization of PVA. These composites could be good candidates for replacing today’s traditional non-biodegradable plastics.