Showing papers in "Thermochimica Acta in 2007"
TL;DR: In this article, various nanoparticles such as multi-walled carbon nanotube (MWCNT), fullerene, copper oxide, and silicon dioxide have been used to produce nanofluids for enhancing thermal conductivity and lubricity.
Abstract: Nanofluid is a kind of new engineering material consisting of nanometer-sized particles dispersed in base fluid. In this study, various nanoparticles, such as multi-walled carbon nanotube (MWCNT), fullerene, copper oxide, and silicon dioxide have been used to produce nanofluids for enhancing thermal conductivity and lubricity. As base fluids, DI water, ethylene glycol, and oil have been used. To investigate the thermo-physical properties of nanofluids, thermal conductivity has been measured. The experimental results of thermal conductivity of nanofluids are compared with the modeling results predicted by Jang and Choi model [14] . The stability of nanofluid has been estimated with UV–vis spectrophotometer. Thermal conductivity of nanofluid has been increased with increasing volume fraction of nanoparticle except for water-based fullerene nanofluid which has lower thermal conductivity than that of base fluid due to its lower thermal conductivity, 0.4 W/mK. Stability of nanofluid has been influenced by the characteristics between base fluid and suspended nanoparticles.
606 citations
TL;DR: In this article, a detailed examination of factors influencing thermal stability, including the role of chemical constitution of organic modifier, composition and structure of nanocomposites, and mechanisms of improvement of thermal stability in polymer/montmorillonite composites, is presented.
Abstract: The results of recent research indicate that the introduction of layered silicate – montmorillonite – into polymer matrix results in increase of thermal stability of a number of polymer nanocomposites. Due to characteristic structure of layers in polymer matrix and nanoscopic dimensions of filler particles, several effects have been observed that can explain the changes in thermal properties. The level of surface activity may be directly influenced by the mechanical interfacial adhesion or thermal stability of organic compound used to modify montmorillonite. Thus, increasing the thermal stability of montmorillonite and resultant nanocomposites is one of the key points in the successful technical application of polymer–clay nanocomposites on the industrial scale. Basing on most recent research, this work presents a detailed examination of factors influencing thermal stability, including the role of chemical constitution of organic modifier, composition and structure of nanocomposites, and mechanisms of improvement of thermal stability in polymer/montmorillonite nanocomposites.
553 citations
TL;DR: In this paper, the thermal conductivities of several nanofluids were consistently greater than the theoretical predictions obtained from existing models, and mechanisms for thermal conductivity enhancement were discussed, where Mono-type nanoparticle suspensions showed the greatest enhancement in thermal conductive, among which the enhancement with CuNPs was the highest.
Abstract: The inherent high thermal conductivity of many nanomaterials has a great potential for enhancing fluidic heat transfer applications. Conductive nanomaterials such as carbon nanotubes (CNTs), copper nanoparticles (CuNPs) and gold nanoparticles (AuNPs), as well as their hybrids such as CNT–CuNP or CNT–AuNP were used in this study to enhance the thermal conductivity of fluids. Mono-type nanoparticle suspensions showed the greatest enhancement in thermal conductivity, among which the enhancement with CuNPs was the highest. Hybrid suspensions did not show the same degree of improvement. The experimentally measured thermal conductivities of several nanofluids were consistently greater than the theoretical predictions obtained from existing models. Mechanisms for the thermal conductivity enhancement are discussed. The stability of nanofluids was estimated by UV–vis–NIR spectrophotometer and it was observed that the stability was influenced by characteristics of nanoparticles.
508 citations
TL;DR: In this paper, the melting behavior of aluminum nanoparticles having an oxide passivation layer was examined using a differential scanning calorimetry (DSC) and the melting point depression, both corrected and uncorrected for the effects of the oxide shell, was linear with the reciprocal of particle radius.
Abstract: The melting behavior of aluminum nanoparticles having an oxide passivation layer is examined using a differential scanning calorimetry (DSC). Both broad and narrow size-distributed particles are studied, and the weight-average particle radius ranges from 8 to 50 nm. With decreasing particle size, the melting response moves towards lower temperatures and the heat of fusion decreases. The effect of the oxide coating on the particles is to apply a compressive force to the aluminum core, thereby increasing the observed melting point and the heat of fusion. The melting point depression, both corrected and uncorrected for the effects of the oxide shell, is linear with the reciprocal of particle radius, as predicted by Gibbs–Thomson equation, although only the corrected data give a value of the solid–liquid interfacial tension comparable to those reported in the literature. The size-dependent heat of fusion is significantly smaller than that predicted by the effects of the surface tension indicating that the solid nanoparticle is at a higher energy than expected, presumably due to the presence of defects or irregularities in the crystal structure at or emanating from the surface. This hypothesis is tested using our data, as well as using data in the literature for tin nanoparticles.
358 citations
TL;DR: In this paper, a cell disrupter generating high power pulses is used for improving the dispersion of nanoparticles, and the transient hot wire method was used for the measurement of thermal conductivity.
Abstract: TiO 2 , Al 2 O 3 , Fe, and WO 3 nanofluids are prepared in a two-step procedure by dispersing nanoparticles in a basefluid. Since nanoparticles form clusters in fluids, a cell disrupter generating high power pulses is used for improving the dispersion of nanoparticles. The transient hot wire method is used for the measurement of thermal conductivity. The thermal conductivities of TiO 2 , Al 2 O 3 , Fe, and WO 3 nanofluids are studied and compared with each other. Nanofluids show a large enhancement of thermal conductivity compared with their basefluids, which exceeds the theoretical expectation of two-component mixture system. We compare thermal conductivities of various nanofluids and discuss the important factors in determining thermal conductivity in this study.
317 citations
TL;DR: In this article, a manufacturing technique based on in situ polymerization is proposed to accomplish the microencapsulation of phase change materials (PCMs) that can ultimately be used in different textile applications, incorporating such products into the multi-component non-conventional fabrics.
Abstract: Thermal comfort of a fabric is one of the important tasks of the designers that require an engineering approach. In this study, we first aimed to establish a manufacturing technique based on in situ polymerization in order to accomplish the microencapsulation of phase change materials (PCMs) that can ultimately be used in different textile applications, i.e. incorporating such products into the multi-component non-conventional fabrics. This method is suitable for the laboratory-scale work as well as the industrial-scale one, considering some important issues like energy and time savings. Four types of polyurea-formaldehyde microcapsules containing different waxes and a control group of 100% shell material were produced and then examined by DSC, particle size and SEM analyses. Furthermore, we also focused on the contributions of microencapsulated PCMs to the thermal performances of fabrics of certain passive insulation characteristics for the given cold weather and physical activity conditions, based on the model developed for a multi-layer fabric system.
301 citations
TL;DR: In this article, the thermal decomposition of Ni(NO3)2·6H2O (1), Ca( NO3) 2·4H 2O (2) and nitryl/nitrosyl nitrato nickelate(II), NO2/NO[Ni(NO 3)3] (3), was investigated by thermogravimetric measurements with quasi-isothermal conditions.
Abstract: The thermal decomposition of Ni(NO3)2·6H2O (1), Ca(NO3)2·4H2O (2) and nitryl/nitrosyl nitrato nickelate(II), NO2/NO[Ni(NO3)3] (3), was investigated by thermogravimetric measurements with quasi-isothermal conditions and compared to Co(NO3)2·6H2O. The respective decomposition processes of 1 and 2 differ from each other showing that at one hand anhydrous Ca(NO3)2 was obtained whereas anhydrous nickel dinitrate has not be formed due to redox and condensation reactions. Instead basic nickel compounds have been formed. In reducing atmosphere nickel metal can be obtained. Anhydrous Ni(NO3)2 results by the thermal degradation of nitryl/nitrosyl nitrato nickelate. FT-Raman spectra have been of help in the judgement of the decomposition processes.
294 citations
TL;DR: In this paper, the basic changes in thermal behavior of different polymeric matrixes (e.g. polyolefins, polyamides, polyvinyl chloride) upon addition of montmorillonite have been described.
Abstract: In previous part of this work factors influencing the thermal stability of polymer nanocomposite materials were indicated, such as chemical constitution of organic modifier, filler content, nanocomposites’ structure and the processing-dependent degree of homogenization of nanofiller, were presented. In this part the basic changes in thermal behaviour of different polymeric matrixes (e.g. polyolefins, polyamides, poly(vinyl chloride) and styrene-containing polymers) upon addition of montmorillonite have been described. Brief description of the kinetics of the decomposition process in inert and oxidative environment, as well as analysis of volatile and condensed products of degradation, have also been presented.
286 citations
TL;DR: In this article, thermal decomposition products evolved during the degradation of several commercially available organoclays (Cloisites TM Na +, 10A, 15A, 20A, 25A, 93A and 30B) were investigated.
Abstract: Thermogravimetry coupled to Fourier transform infrared spectroscopy (TGA/FTIR) has been used to study the thermal decomposition products evolved during the degradation of several commercially available organoclays (Cloisites TM Na + , 10A, 15A, 20A, 25A, 93A and 30B). It was found that the decomposition pattern of the organoclays was different for each sample: Cloisite TM 10A shows three well-defined degradation stages, Cloisite TM 30B only two stages and the Cloisite TM 93A only one weight loss; Cloisites TM 15A, 20A and 25A exhibited a more complex behavior showing one main stage and a shoulder. It was also observed that the onset of the decomposition was different for each type of organoclay, being Cloisite TM 10A the lowest (160 ◦ C) and Cloisite TM 93A the highest (212 ◦ C). FTIR analysis of the evolved products from their non-oxidative thermal degradation showed the release of water, aldehydes, carboxylic acids, aliphatic compounds and, in some cases, aromatic compounds and CO2. It is suggested that the degradation of both tallow residue and unexchanged surfactant explain the presence of some products evolved during degradation of organoclays. © 2007 Elsevier B.V. All rights reserved.
263 citations
TL;DR: In this paper, an ab initio quantum chemical calculations (DFT-B3LYP) was used to predict thermal decomposition mechanisms, temperatures and the activation energies of the thermal breakdown reactions.
Abstract: The long-term thermal stability of ionic liquids is of utmost importance for their industrial application. Although the thermal decomposition temperatures of various ionic liquids have been measured previously, experimental data on the thermal decomposition mechanisms and kinetics are scarce. It is desirable to develop quantitative chemical tools that can predict thermal decomposition mechanisms and temperatures (kinetics) of ionic liquids. In this work ab initio quantum chemical calculations (DFT-B3LYP) have been used to predict thermal decomposition mechanisms, temperatures and the activation energies of the thermal breakdown reactions. These quantum chemical calculations proved to be an excellent method to predict the thermal stability of various ionic liquids.
229 citations
TL;DR: The exoskeletons of the American lobster Homarus americanus and of the edible crab Cancer pagurus were analyzed with structural and chemical methods as mentioned in this paper, showing that the shell of the lobster is less mineralized (and therefore lighter and less hard) than that of the crab.
Abstract: The exoskeletons of the American lobster Homarus americanus and of the edible crab Cancer pagurus were analysed with structural and chemical methods. The exoskeletons consist of crystalline magnesian calcite in the form of nanocrystals (domain size about 20 nm), amorphous calcium phosphate (ACP), and-chitin. The composition varies among different parts of the skeleton and also between the two species. Differences are related to the mechanical requirements and biological escape behaviour of the animals. The finger and claw are strongly mineralized and very hard. The shell of the body (the carapace) is less mineralized and more elastic. The lobster, as a mobile, fast-swimming animal, typically escapes from a predator whereas the crab clings to the ground and burrows into the sand. Consequently, the shell of the lobster is less mineralized (and therefore lighter and less hard) than the shell of the crab. © 2007 Elsevier B.V. All rights reserved.
TL;DR: In this article, the pyrolysis combustion flow calorimeter (PCFC) was used as a tool for assessing the flammability of a polycarbonate (bisphenol A)/acrylonitrile-butadiene-styrene (PC/ABS) blend containing different flame retardants and additives.
Abstract: The pyrolysis combustion flow calorimeter (PCFC) as a tool for assessing the flammability of a polycarbonate (bisphenol A)/acrylonitrile–butadiene–styrene (PC/ABS) blend containing different flame retardants and additives was investigated. Strategies are proposed for analysing multi-step decomposition. The heat release capacity (HRC) and total heat release (HR), obtained by PCFC, are related to the char yield and the heat of complete combustion of the volatiles. Physical affects such as dripping, wicking, and sample thickness are not described, nor are chemical effects such as flame inhibition because pyrolysis and combustion are forced to completion on a small (milligram) sample. Varying the combustion temperature or oxygen concentration results in incomplete combustion as occuring in real fires. The correlations with flammability (UL 94, LOI) and forced flaming combustion in a cone calorimeter are discussed. The best correlation is found between HR and LOI. Reasonable correlation exists between HRC and char residue with the LOI and for HRC and HR with peak heat release rate (pHRR) in the cone calorimeter. Combining results from PCFC with those from oxygen bomb or cone calorimeter tests yields an additional understanding of fire behaviour.
TL;DR: In this paper, the thermal conductivity of boron nitride (BN) reinforced high density polyethylene (HDPE) composites was investigated under a special dispersion state of BN particles in HDPE, i.e., BNs particles surrounding HDPE matrix particles.
Abstract: Thermal conductivity of boron nitride (BN) reinforced high density polyethylene (HDPE) composites was investigated under a special dispersion state of BN particles in HDPE, i.e., BN particles surrounding HDPE matrix particles. The results indicated that the special dispersion of BN in matrix gives the composites high thermal conductivity at low filler content; moreover, the smaller BN particles can more easily form conductive chains of filler compared to the larger filler particles. Examining the dependence of electrical insulation and mechanical properties of the composites on BN content demonstrated that the reinforced composites containing 30% by volume of filler has good electrical insulation and mechanical properties.
TL;DR: In this paper, the authors quantitatively measured the heat capacity of pure dry silk fibroin films cast from water solution, and containing bound water, and found that the total solid specific heat capacity is in good agreement with the calculated solid baseline in the low-temperature region below about 240 K.
Abstract: Silk fibroin films cast from water solution, and containing bound water, are quantitatively studied in this work. First, to obtain the solid and liquid heat capacities of the pure dry silk fibroin, cyclic heat treatment was used to monitor the process of removing the bound water. After water removal, the glass transition of pure non-crystalline silk was observed at 451 K (178 °C). The solid and liquid heat capacities of the pure silk fibroin were then measured using differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), and quasi-isothermal TMDSC, and found to be: C p ( T ) solid = 0.134 + 3.696 × 10 −3 T J/g K and C p ( T ) liquid = 0.710 + 3.47 × 10 −3 T J/g K over the temperature region from 200 to 450 K. These heat capacities were used to construct the underlying baseline heat capacity for the combined silk–water system. When the combined silk–water system is studied, bound water is lost from the film during heating, and the loss of mass is quantified using thermogravimetric analysis (TGA). Bound water in the silk film acts as a plasticizer, and a lower glass transition of the silk–water system is observed. Comparison of the measured heat capacity of the silk–water system to the calculated total baselines was made in the vicinity of the water-induced glass transition. Results show that the total solid specific heat capacity is in good agreement with the calculated solid baseline in the low-temperature region below about 240 K. As temperature increases above the lower glass transition, all bound water eventually leaves the silk, and the free volume and the silk mobility are reduced. This allows the upper glass transition of the dried silk to be observed.
TL;DR: In this paper, the thermal degradation of chitosan, degree of acetylation 12%, was studied by using TG in nitrogen atmosphere in isothermal and dynamic conditions, with an apparent activation energy of 149.6 and 138.5 kJ/mol, respectively.
Abstract: The thermal degradation of chitosan, degree of acetylation 12%, was studied by using TG in nitrogen atmosphere in isothermal and dynamic conditions. Temperatures ranging from 240 °C to 280 °C were studied in isothermal conditions while heating rates in the interval 2.5–15.0 °C/min were employed when using dynamic conditions. The data issued from the dynamic experiments were treated by the methods proposed by Ozawa–Flynn–Wall and Kissinger, resulting in apparent activation energy of 149.6 kJ/mol and 138.5 kJ/mol, respectively. The approach proposed by MacCallum was used to treat the data issued from the isothermal experiments, resulting in E a = 153 kJ/mol in good agreement with the results issued from the dynamic experiments. The use of the isoconvertional method due to Vyazovkin also showed a good agreement with the value of the activation energy issued by applying the method of Ozawa–Flynn–Wall. It was also observed that regardless of using isothermal or dynamic experimental conditions, the kinetic model best suited to describe the thermal degradation of chitosan is the catalytic Sestak–Berggren model.
TL;DR: In this article, two paraffin waxes acting as phase change materials were directly incorporated into the polyurethane foams at different ratios, and polymerization modified by means of n-alkane addition could be achieved without any adverse effect.
Abstract: Thermal energy storage plays an important role in heat management because of the demand for developed energy conservation, and has applications in diverse areas, from building heating/cooling systems which enable solar energy incorporation into the structure, to textiles and clothings providing an enhanced thermal comfort. In this study, we aimed to improve thermal characteristics of polyurethane rigid foams that have been widely used for thermal insulation as the ultimate energy savers due to their ability to form sandwich structures with various facer materials. Through a laboratory-scale work, two paraffin waxes acting as phase change materials, namely n-hexadecane and n-octadecane, each of which is capable of managing large heat storage/release, were directly incorporated into the polyurethane foams at different ratios. Polymerization modified by means of n-alkane addition could be achieved without any adverse effect. In order to determine both structural and thermal characteristics, seven types of foams produced were examined by FT-IR, SEM, DSC analyses, calorimeter bomb and mechanical tests. Results show that polyurethane foams can be designed as thermal insulators equipped with an improved buffering function against temperature changes.
TL;DR: In this article, the junction temperatures of high power LED arrays with and without heat pipe at the same air velocity of 7 m/s were reported to be 87.6 °C and 63.3 °C, respectively.
Abstract: This paper reports on thermal characterization of high power LED arrays. Thermal transient methods are used to measure the junction temperature and calculate the thermal resistance. The emphasis is placed upon the investigation of junction temperature rise of LED array for a limited range of boundary conditions which include design effect of heat pipe, convection condition, and ambient temperature. The junction temperatures of LED array with and without heat pipe at the same air velocity of 7 m/s were 87.6 °C, and 63.3 °C, respectively. The corresponding thermal resistances of LED array were measured to be 1.8 K/W and 2.71 K/W. It was found out that the measured junction temperatures and thermal resistance of LED array are increased with the input power and ambient temperature, but decreased with the air velocity. An analytical thermal model analogous with an equivalent parallel circuit system was proposed and was verified by comparison with experimental data.
TL;DR: In this article, the authors analyzed the dehydration kinetics of equilibrium swollen poly (acrylic acid) hydrogel by both model-fitting and model-free approaches and found that the values of Arrhenius parameters obtained in such a way are in fact an average that does not reflect changes in the reaction mechanism and kinetics with the extent of conversion.
Abstract: The dehydration kinetics of equilibrium swollen poly (acrylic acid) hydrogel is analyzed by both model-fitting and model-free approaches. The conventional model-fitting approach assuming a fixed mechanism throughout the reaction and extract a single values of the apparent activation energy (Ea) and pre-exponential factor (A) and was found to be too simplistic. The values of Arrhenius parameters obtained in such a way are in fact an average that does not reflect changes in the reaction mechanism and kinetics with the extent of conversion. The model-free approach allows for a change of mechanism and activation energy, Ea, during the course of a reaction and is therefore more realistic. The complexity of the dehydration of poly (acrylic acid) hydrogel is illustrated by the dependence of Ea and A on the extent of conversion, α (0.05 ≤ α ≤ 0.98). Under non-isothermal conditions, Ea decreases with α for 0 ≤ α ≤ 0.50, followed by an approximately constant value of Ea during further dehydration. For 0 ≤ α ≤ 0.50, dehydration is complex, which probably involving a combination of several processes. In the constant-Ea region, non-isothermal dehydration follows the three-dimensional phase boundary model (R3). The complex hydrogen-bond pattern in poly (acrylic acid) hydrogel is probably responsible for the observed dehydration behavior. An existence of compensation effect is accepted and explanation of compensation effect appearance during the hydrogel dehydration is suggested.
TL;DR: In this article, the thermal behavior of a mixture containing an ammonium polyphosphate based compound (AP760) and zinc borate (ZB) was investigated by thermogravimetry.
Abstract: The thermal behaviour of a mixture containing an ammonium polyphosphate based compound (AP760) and zinc borate (ZB) is investigated. After an investigation of the degradation of the pure components, the interactions between them are examined by thermogravimetry. Then, X-ray diffraction (XRD) and 11B and 31P solid-state nuclear magnetic resonance (NMR) measurements have been carried out on residues of mixtures of AP760 and FBZB heat treated at different characteristic temperatures. It reveals the nature of the interactions taking place between the two components. It is demonstrated that reactions lead to the formation of zinc phosphate and of borophosphates. Mechanisms of thermal degradation are proposed.
TL;DR: In this article, the activation energies (E a ) were obtained from these methods and compared, and it was found that the values of E a for modified epoxy hybrids are higher than that of pure epoxy.
Abstract: Hybrids containing silicon, phosphorous and nitrogen were prepared by the sol–gel method and compared with pure epoxy The silicon, phosphorous and nitrogen components were successfully incorporated into the networks of polymer Thermogravimetric analysis (TGA) was used for rapid evaluation of the thermal stability of different materials The integral procedure decomposition temperature (IPDT) has been correlated the volatile parts of polymeric materials and used for estimating the inherent thermal stability of polymeric materials The IPDT of pure epoxy was 464 °C and the IPDTs of hybrids were higher than that of pure epoxy The thermal stability of hybrids increased with the contents of inorganic components The inorganic components can improve the thermal stability of pure epoxy Two methods have been used to study the degradation of hybrids containing silicon, phosphorous and nitrogen hybrid during thermal analysis These investigated methods are Kissenger, Ozawa's methods The activation energies ( E a ) were obtained from these methods and compared It is found that the values of E a for modified epoxy hybrids are higher than that of pure epoxy The hybrids of high activation energy possess high thermal stability
TL;DR: In this article, the NETZSCH LFA 457 MicroFlash was developed for measurement of the thermophysical properties (thermal diffusivity, specific heat, thermal conductivity).
Abstract: The flash method is a well-known technique for measurement of the thermophysical properties (thermal diffusivity, specific heat and thermal conductivity) of solid materials. Easy sample preparation, fast measurement times and high accuracy are only some of the advantages of this non-contact, non-destructive testing technique. A new laser flash system, the NETZSCH LFA 457 MicroFlash, was developed for measurement of the thermophysical properties (thermal diffusivity, specific heat, thermal conductivity). The system can be equipped with two different user-interchangeable furnaces allowing measurement between −125 and 1100 °C. The vacuum-tight construction of the system allows tests under well-defined atmospheres as well as under vacuum. The integrated automatic sample changer enables testing of up to three samples at the same time. Presented in this work are the technical details of the instrument and some performance tests as well as various application examples on industrially relevant materials.
TL;DR: In this article, a structural model of a grafting process of the interlayer [Al(OH)4]− tetrahedron onto hydroxylated octahedrons of aluminum-oxide polyhedrons has been proposed in order to explain observed loss of one water molecule, shrinkage of interlayer spacing and qualitative changes of FT-IR spectra.
Abstract: Thermal dehydration of dicalcium aluminate hydrate, C2AH8, has been investigated by simultaneous differential thermal and thermo gravimetric analysis (DTA/TGA), powder X-ray diffraction (XRD), temperature-dependent infrared spectroscopy (FT-IR), and BET method of surface area measurement. The temperature-dependent infrared measurements were studied by two-dimensional infrared (2D-IR) correlation spectroscopy. The structure of aluminum-oxide polyhedron, characterized by 27Al solid state NMR spectrum method and FT-IR, shows tetrahedron and octahedron as the main forms of aluminum-oxide polyhedrons in C2AH8 sample. From the results obtained a variety of structural transformations observed are explained as a consequence of the removal of loosely held interlayer water molecules at lower temperatures, followed by grafting process of the interlayer [Al(OH)4]− anion. Structural model of a grafting process of the interlayer [Al(OH)4]− tetrahedron onto hydroxylated octahedrons of [Ca2Al(OH)6]+ layers has been proposed in order to explain observed loss of one water molecule, shrinkage of interlayer spacing and qualitative changes of FT-IR spectra. At higher temperatures the dehydroxylation of the lattice and decomposition of the interlayer species occurs, yielding amorphous material that crystallizes into C3A and C12A7 at 885 °C. Those findings provide improvement in the interpretation of thermo-analytical results of calcium aluminate cements (CAC) hydration products, and better understanding of CAC conversion process.
TL;DR: In this article, the effect of EG content and preparation methods on thermal conductivity of composites were investigated, and it was found that with increasing EG, the thermal conductivities increased, and solution intercalation method showed better effect than melt mixing method in improving the thermalconductivity of composite.
Abstract: Silicone/expanded graphite (EG) composites were prepared by melt mixing and solution intercalation. The effect of EG content and preparation methods of composites on thermal conductivity of composites were investigated, and it was found that with increasing EG, the thermal conductivity increased, and solution intercalation method showed better effect than melt mixing method in improving the thermal conductivity of composites. Dynamic mechanical analysis (DMA) was used to reveal that the storage modulus (E′) of SI-prepared composite is relatively greater than those of MM-prepared composite counterparts. The scanning electron microscopes (SEM) observation confirmed that striking increase of thermal conductivity might be attributable to higher surface-to-volume ratio of EG fillers.
TL;DR: In this article, the authors investigated polyvinylidene fluoride (PVDF) in non-isothermal mode at ultra-high cooling rates ranging between 30-3000 k/s as well as at constant temperatures after quenching at 6000k/s.
Abstract: Melt-crystallization of polyvinylidene fluoride (PVDF) was investigated in non-isothermal mode at ultra-high cooling rates ranging between 30–3000 K/s as well as at constant temperatures after quenching at 6000 K/s. An increase of the cooling rate above 150 K/s leads to the formation of β phase manifested by a low temperature shoulder of crystallization exotherm in addition to the α modification. At the cooling rates above 2000 K/s there is only low temperature exothermic peak that is attributed to the crystallization of pure β modification. Isothermal crystallization was possible to realize at 110 °C as the lowest, resulting in α form. Much higher crystallization rate in submicrogram samples, as compared to standard DSC experiments, is also reported.
TL;DR: In this paper, the kinetic and thermodynamic characterization of a non-catalyzed reaction between poly(hexamethylene carbonate- co-caprolactone)diol (PHMC- co -PCL) with a stochastic functional concentration, using both isothermal and dynamic differential scanning calorimetry, DSC, as well as Fourier transform infrared spectroscopy, FT-IR, was presented.
Abstract: This paper presents the kinetic and thermodynamic characterization of a non-catalyzed reaction between poly(hexamethylene carbonate- co -caprolactone)diol (PHMC- co -PCL) and aliphatic hexamethylene diisocyanate (HDI) with a stochiometric functional concentration, using both isothermal and dynamic differential scanning calorimetry, DSC, as well as Fourier transform infrared spectroscopy, FT-IR. DSC data were fitted using a Kamal autocatalytic equation. Model-free-isoconversional methods were also applied to analyse the conversion dependence of the global activation energy. This relation was used to predict the reaction conversion versus time pattern at different temperatures and to compare it with that of the model approach. Kinetic modelling and model-free analysis successfully described the conversion versus time curves. The reaction can be divided in two different paths: the forward path and the autocatalyzed one. Results corroborated that autocatalysis is promoted by the urethane group. Activation energies for both reaction paths have been found to be higher than those presented in the literature for aromatic diisocyanate systems, which explains the lower reaction rate of the presented system.
TL;DR: In this article, a twin-screw co-rotating extruder was used to extract isotactic polypropylene (iPP) and carbon nanoparticles (CN) from carbon black nanoparticles.
Abstract: Nanocomposites of isotactic polypropylene (iPP) and carbon nanoparticles (CN) were prepared using a twin-screw co-rotating extruder. Morphological characterization of the prepared materials was carried out by TEM and confocal micro-Raman. Viscoelastic and thermal properties were also determined by DMA and DSC measurements. Storage modulus was improved while the T g was slightly increased. A small increase of melting temperature was obtained while crystallization temperatures were slightly altered. Degradation kinetics was investigated by thermogravimetric analysis (TGA and DTG). The nanoparticles’ presence caused a shift of the onset mass loss temperature to higher temperatures. The activation energies were calculated using the isoconversional methods of Ozawa, Flynn and Wall (OFW) and Friedman and were found to be significantly enhanced by the presence of carbon black nanoparticles. Degradation took place in two autocatalysis stages, the first corresponding to a small initial mass loss, while the second, where substantial mass loss took place, was attributed to the main decomposition mechanism.
TL;DR: In this article, the crystallization behavior of poly(ethylene oxide) (PEO)/multi-walled carbon nanotubes (MWCNT) and PEO/chemically modified MWCNT nanocomposites were investigated by means of differential scanning calorimetry.
Abstract: Crystallisation behaviour of poly(ethylene oxide) (PEO)/multi-walled carbon nanotubes (MWCNT) and PEO/chemically modified MWCNT nanocomposites were investigated by means of differential scanning calorimetry. Non-isothermal crystallisation experiments showed that incorporation of MWCNT and chemically modified MWCNT reduced the crystallinity and restricted the spherical crystal growth of PEO. The nucleation sites decrease and spherical crystal size increased compared to the neat PEO. Change of crystal structure from spherical to disk-like was revealed by Avrami equation when MWCNT was added up to 1 wt.%.
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TL;DR: In this article, a review about the application of calorimetry to study soil properties and its metabolism is presented, and the contributions of these techniques to different topics in soil research are described and their importance for the environmental concern is discussed in the light of this new era.
Abstract: This paper is a review about the application of calorimetry to study soil properties and its metabolism. Although this research has increased slowly but continuously during the last 30 years, it is true that it has received poor attention. One reason for that could be the complexity of the soil and the difficulties to investigate it from a thermodynamic point of view. In this paper we would like to demonstrate that calorimetry constitutes a very suitable method to face the main topics related to soil quality and activity. Very well known indicators used in soil research can be measured by different thermal and calorimetric methods such as differential scanning calorimetry (DSC), thermogravimetry (TG) and isothermal calorimetry (ITC). TG and DSC are both temperature scanning methods used for estimation of certain properties of the soil material such as organic matter, ignition temperature, humification index, quartz content, so on, whereas the study of the soil microbial metabolism is conducted under essentially isothermal conditions by ITC. In this review, the contributions of these techniques to different topics in soil research are described and their importance for the environmental concern is discussed in the light of this new era.
TL;DR: In this paper, the occupancy of cis and trans positions of the dioctahedral smectites was investigated by simultaneous thermal analysis, and limits for the proportion of cis - and trans -vacancies were defined resulting in four classes (cv, cv/tv; tv/cv; tv) for montmorillonites with respect to the distribution of the octahedral cations.
Abstract: Montmorillonites are structural and chemical heterogenic with the octahedral sheet being the most distinguished structural feature. The occupancy of cis - and trans -positions of the dioctahedral smectites was investigated by simultaneous thermal analysis. Cis -Vacant (cv) phyllosilicates dehydroxylate between 650 and 700 °C and trans -vacant (tv) varieties between 500 and 550 °C. Calculation of peak areas from the peak fitted evolved water curves were used to estimate the ratio of trans - to cis -vacancies. Limits for the proportion of cis - and trans -vacancies were defined resulting in four classes (cv; cv/tv; tv/cv; tv) for montmorillonites with respect to the distribution of the octahedral cations. Most common are mixed cv/tv montmorillonites with dominating amounts of cv 2:1 layers (74–50%). Pure cv montmorillonites occur in nature. Some montmorillonites classified as cv contain up to 25% tv layers. Pure tv montmorillonites were not found. All tv montmorillonites contain cv 2:1 layers (up to 24%).
TL;DR: In this paper, thermal analysis of three kinds of ceramic package designs for high power LEDs was made by transient thermal measurement and thermal simulation using the Finite Volume Method, and the results showed that the junction temperature decreases with the effective contact area ratio in the LED packages.
Abstract: In this paper we present thermal analysis of three kinds of ceramic package designs for high power LEDs. The analysis was made by transient thermal measurement and thermal simulation using the Finite Volume Method. The three ceramic packages under investigation employ same configuration of GaN-based chip, but they have different size and distribution of thermal vias. Three designs of LED packages resulted in significantly different thermal behaviors. Thermal behaviors, described as thermal resistance, of the three packaging designs were compared and evaluated as functions of bulk thermal resistance, spreading resistance, and surface roughness. The deviation between the simulated results and measured data were attributed to the different surface roughness in the interfaces between the packaging components. It was demonstrated that the junction temperature decreases with the effective contact area ratio in the LED packages.