Showing papers in "Journal of Thermal Analysis and Calorimetry in 2015"

An experimental study on the effect of diameter on thermal conductivity and dynamic viscosity of Fe/water nanofluids

Abstract: The addition of nanoparticles to a base fluid is one of the significant issues to enhance heat transfer. In this study, different nanofluids were developed by mixing a water base fluid with magnetic nanoparticles. Thermophysical properties such as thermal conductivity and viscosity of the obtained nanofluid were investigated. The effect of different nominal diameters of nanoparticles and concentrations of nanoparticles on the thermal conductivity and viscosity of nanofluids have been examined. Three different diameters of magnetic nanoparticles (about 37 nm, 71 nm, and 98 nm) have been tested in this experimental investigation. Experimental results indicate that thermal conductivity increases as volume fraction increases, and thermal conductivity of the nanofluid increases with a decrease of nanoparticle’s size. Moreover, the nanofluid dynamics viscosity ratio increases with an increase in particle concentration and nanoparticle’s diameter. This paper identifies several important issues that should be considered in future work.

Evaluation of thermal conductivity of COOH-functionalized MWCNTs/water via temperature and solid volume fraction by using experimental data and ANN methods

Abstract: In the present study, the results of several experiments have been used to obtain the thermal conductivity of multi walls carbon nanotubes-water nanofluid. For this purpose, COOH-functionalized MWCNTs nanoparticles have divided into different solid volume fractions in order to disperse in water as the base fluid by different dispersion methods. The thermal conductivity measurement applied in different solid concentrations, up to 1 %, and at the temperatures ranging from 25 to 55 °C. In this paper, based on the experimental data, a new correlation for predicting the thermal conductivity of COOH-functionalized MWCNTs/water nanofluid proposed. After that, for simulating the thermal conductivity of this nanofluid, the artificial neural network is used. For this purpose, multilayer percepetron neural network is used. The network input variables are temperature and solid volume fraction, and the network output variable is thermal conductivity. The results extracted from the artificial neural network show good agreement with the experimental data. The mean square error value is 4.04E−06 that shows excellent performance of artificial neural network to predict thermal conductivity of COOH-functionalized MWCNTs/water nanofluid.

Turbulent forced convection heat transfer and thermophysical properties of Mgo–water nanofluid with consideration of different nanoparticles diameter, an empirical study

Abstract: In the present paper results of an experimental study on thermal conductivity, dynamic viscosity and Nusselt number of turbulent forced convection of Magnesium Oxide–water nanofluid in a circular straight pipe is presented. The considered pertinent parameters are Reynolds number, nanoparticles volume fraction and nanoparticles diameter. The pure water and nanofluid with the nanoparticle volume fraction of 0.005, 0.01, 0.015, 0.02 and the nanoparticles diameter of 60, 50, 40 and 20 nm are considered. The experimental values of the thermal conductivity and the dynamic viscosity shows that traditional formulas underestimates these thermophysical parameters. Also the experimental results indicates that the existence of the nanoparticles in the pure water with all considered values of the nanoparticles volume fraction and diameter motivates the rate of heat transfer to increase. The nanofluids with higher volume fraction and smaller nanoparticles diameter results in higher Nusselt number.

Thermal conductivity and viscosity of Mg(OH) 2 -ethylene glycol nanofluids

Abstract: A series of experiments have been carried out to determine the thermal conductivity and viscosity of a novel nanofluid, i.e., Mg(OH)2/ethylene glycol (EG). The thermal conductivity and viscosity of nanofluids with volume fractions by 2 % in the temperature range of 25–55 °C are measured. The results unfold that in the temperature of 35 °C, called critical temperature, the ratio of nanofluid viscosity to water viscosity is minimized. This critical temperature reveals that it is more advantageous to use Mg(OH)2/EG nanofluids instead of water at temperatures higher than 35 °C from the pressure drop and pumping power viewpoint.

Investigation on the thermal hazards of 18650 lithium ion batteries by fire calorimeter

Abstract: In applications of lithium ion batteries, it is a requisite to precisely appraise their fire and explosion hazards. In the current study, a fire calorimeter is utilized to test the combustion performance of two commercial 18650 lithium ion batteries (LiCoO2 and LiFePO4) at different state of charge (SOC). Characteristics on thermal hazards of lithium ion batteries including surface temperature, time to ejection, mass loss, and heat release rate (HRR) are measured and evaluated. In case of thermal runaway, all the lithium ion batteries will rupture the can and catch fire even explode automatically. The solid electrolyte interface layer decomposition and the polymer separator shrinking are direct causes of the lithium ion battery fire. The experimental results show that the HRR and total heat generally rise as the SOC increases, whereas the time to first ejection and the time gap between first and second ejection decrease. LiCoO2 18650 battery shows higher explosion risk than LiFePO4 18650, as the former has released much more oxygen. The experimental combustion heats calculated and modified in the oxygen consumption method reveal that the internally generated oxygen have significant effect on the estimate of the heat, where the largest modified rate is 29.9 for 100 % SOC LiCoO2 18650 battery. The results can provide scientific basis for fire protection during the storage and distribution of lithium ion batteries.

Thermal behavior of natural dolomite

Abstract: The thermodynamic analysis of the thermal decomposition reaction of carbonates showed that the temperatures of thermal stability of dolomite and magnesium carbonate are very similar. Kinetics of isothermal and non-isothermal decompositions of natural dolomite in air atmosphere were investigated. Under the examined conditions, dolomite decomposed in a one-stage process. The final solid products of dolomite’s decomposition were calcium and magnesium oxides. The average value of isoconversional apparent activation energy of non-isothermal decomposition was 205.60 kJ mol−1. The kinetics of decomposition can be described by the power-law model P2/3 with ln(A) = 20.98 for the extent of reaction α 0.77.

A comprehensive review of last experimental studies on thermal conductivity of nanofluids

Abstract: In this paper, the last experimental results of the thermal conductivity of nanofluids have been investigated; therefore, because of the enormous numbers or some repetitions of such studies, it just tried to focus on those which had more acceptable results; however, it is not intended to present a systematic summary of the available references from the literature. All kinds of nanoparticles such as Al2O3, Fe3O4, TiO2, and CuO and their base fluids such as water, engine oil, glycol, and ethylene glycol have been investigated. The results showed that the effect of particle size of nanoparticle has a reversed impact on the value of thermal conductivity. To clarify, the thermal conductivity could increase by reduction in size of nanoparticles. On the other hand, the lower the suspension volume fraction is, the higher the thermal conductivity will be in nonlinear relation. Also the thermal conductivity and the temperature of the suspension have the direct influence on each other. In addition to the fact that a review paper has been prepared, the belief is to start this research with the scientific classification in order of all terms and expressions which have been used by previous scholars.

Assessment of polyamide-6 crystallinity by DSC

Abstract: This study addresses the question of the crystallinity determination of PA6 by means of DSC in the case when structural changes occur over a very large temperature domain during the heating scan. The temperature dependence of the melting enthalpy is then of crucial importance for determining the amount of crystalline phase involved in the various processes, and thus the initial crystallinity. Both DSC and WAXS measurements have been carried out of a PA6 sample submitted to various thermal treatments in order to identify the crystalline forms and the temperature-induced structural changes. The melting enthalpy dependence on temperature of PA6 was computed from heat capacity data of the solid and liquid borrowed from literature data tables. Similar computations were performed for PA66 which is likely to exhibit analogous structural changes during DSC analysis.

On the thermal decomposition of nesquehonite

Abstract: Among the phases in the MgO–CO2–H2O system, nesquehonite, MgCO3 ·3H2O, attracts particular attention because of its potential application to carbon capture. However, its stability and the reported sequence of phases formed during the course of its thermal decomposition differ between authors and the corresponding decomposition mechanisms find various explanations. To improve the knowledge on the thermal decomposition of nesquehonite, new thermal data are presented and X-ray diffraction was used to follow the evolution of the solid products of decomposition. During thermal decomposition, nesquehonite loses its water below 300–350 °C whereas CO2 is lost above 300–350 °C, but the mechanism of thermal decomposition process is influenced by the choice of experimental conditions. The first loss of water, between ~55 and ~135 °C, results in a partial collapse of the nesquehonite structure and in the formation of a crystalline phase which is referred to as “phase X” (approximately MgCO3·2H2O) and this gradually converts into an amorphous phase upon further heating and water loss. The regeneration of nesquehonite upon rehydration of either “phase X” or the amorphous phase suggests that sufficient structural elements persist throughout the initial stages of decomposition to reconstitute nesquehonite.

Thermal decomposition of d-metal nitrates supported on alumina

Abstract: The thermal decomposition of cobalt, nickel, manganese, zinc, and copper nitrates supported on nanometric alumina was investigated and compared with decomposition of corresponding bulk nitrates TG, DTA, and MS measurements in air were performed The supported nitrates decompose in lower temperatures than the bulk ones and their decomposition proceeds in fewer stages which are better separated Synthesized materials and bulk nitrates before degradation of nitrates groups undergo dehydration For decomposition of manganese and copper nitrates, the last step of water vapour releasing is combined with degradation of nitrate groups thus formation of anhydrous metal nitrate during decomposition is not achievable Thermal decomposition of bulk nitrates leads to oxides—Co3O4, NiO, MnO2, ZnO, and CuO—respectively, as the solid residue The nickel, zinc, copper, and manganese nitrates while supported on alumina decompose to corresponding oxides (NiO, ZnO, CuO, MnO2) as well For decomposition of cobalt nitrate while supported on Al2O3 as the solid residue CoAl2O4 were identified The correlation between dehydration and degradation of nitrates groups temperatures for bulk and supported nitrates was analysed in terms of atomic properties of d-metals

Thermal stability of magnetic nanoparticles coated by blends of modified chitosan and poly(quaternary ammonium) salt

Abstract: Magnetite nanoparticles (Fe3O4 NPs) coated by modified chitosan (m-CS) rich of amino groups and its mixture with poly[N-benzyl-2-(methacryloxy)-N,N-dimethylethanaminium bromide] (PQ) with various mass ratios were prepared by co-precipitation method. The thermal stability of magnetic materials has been investigated by thermogravimetric analysis (TG/DTG/DTA) in air and nitrogen atmosphere. It was found that magnetic NPs coated by polymer blend were characterized by lower thermal stability than protected only by alone CS or m-CS. Magnetite NPs enhance thermooxidative degradation of polymers, however, the stable surface layer, formed from polymer degradation products, efficiently protects the magnetite transformation at higher temperatures. The content of magnetite in obtained systems has been also determined from TG curves.

Hydration kinetics of composite binder containing slag at different temperatures

Abstract: Hydration heat evolution rate and cumulative hydration heat of cement–slag blends were measured at 298, 318 and 333 K by isothermal calorimeter. Based on hydration kinetics model, three hydration processes of composite binder, namely nucleation and crystal growth (NG), interactions at phase boundaries (I) and diffusion (D), were characterized, and the relationship between hydration rate and hydration degree of binder was discussed in individual periods. The kinetics exponent, n, rate constant, K, and apparent activation energy, E a, of hydration were calculated and analyzed. Results indicated that the kinetics model could simulate the hydration process of composite binder containing no more than 70 % of slag, whose process sequence is NG → I → D at 298 and 318 K, but it becomes NG → D at 333 K. Composite binder containing 90 % of slag cannot use this model due to the variation in mechanism. The simulation error has a certain increase as increasing temperature and slag content. E a increases with an increase in slag content.

DSC and thermal imaging studies of methane hydrate formation and dissociation in water emulsions in crude oils

Abstract: The processes of methane hydrate formation from 50 mass% water-in-oil emulsion from the Mamontovskoe, Sovetskoe, Van-Eganskoe, and Vakhskoe oil deposits (East Siberia) were studied using differential scanning calorimetry. It was shown that different types of thermal effects may be observed in curves corresponding to the formation of hydrate (and/or ice) in different emulsions. In particular on the curves, we observed the appearance of a single asymmetric thermal effect or a set of a large number of separate small thermal effects. The analysis of the results allowed us to conclude that hydrate formation (ice freezing) in the studied emulsions occurs as a collective process, which involves a lot of water droplets in the emulsions. This process occurs in some space inside the emulsion sample. The volume of this space determines the type of curve recorded in experiment. Secondary nucleation (nucleation of the solid phase due to the contact of a drop of water with the neighboring hydrate or ice particle) enables fast formation of gas hydrate in all droplets inside this space.

Thermal characterization of the promising energetic material TKX-50

Abstract: The thermal behavior of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) was measured by differential scanning calorimetry (DSC), thermal gravimetric analysis–Fourier transform infrared spectroscopy (TG-FTIR) and thermal gravimetric analysis–differential scanning calorimetry–mass spectrometry (TG-DSC-MS). The critical temperature of thermal explosion (T b) and the initial decomposition temperature (T p0) were determined to be 523.39 and 513.80 K, respectively. The apparent activation energy (E) and pre-exponential factor (A) of the exothermic decomposition reaction, and activation entropy (ΔS ≠), activation enthalpy (ΔH ≠), activation Gibbs free energy (ΔG ≠) at T P0 of the reaction and the critical temperature of thermal explosion (T b) were calculated: E K = 237.59 kJ mol−1, E O = 234.11 kJ mol−1, ΔS ≠ = 209.83 J K−1 mol−1, ΔH ≠ = 233.32 kJ mol−1, ΔG ≠ = 126.53 kJ mol−1. TG-DSC-FTIR/MS analysis of thermal decomposition products of TKX-50 reveals that the main decomposition products are N2, H2O, NH3, NH2, N2O and NO.

Thermal characterization of HITEC molten salt for energy storage in solar linear concentrated technology

Abstract: The enhancement in the storage systems developed by solar thermoelectric centrals brings to this renewable energy a considerable efficiency increase. This improvement propitiates the design of storage fluids with lower melting point and higher thermal stability such as molten salt mixtures. This research has broadly studied the HITEC mixture composed by 53 mass% KNO3 + 40 mass% NaNO2 + 7 mass% NaNO3, with the aim to improve the existing solar salt used as energy storage fluid in CSP plants and focus the thermal properties obtained for application in solar linear concentrated technology. HITEC molten salt shows better physicochemical properties than the binary solar salt (60 % NaNO3 + 40 % KNO3), due to its lower melting point which can improve the work temperature range in commercial solar plants. The tested properties studied by differential scanning calorimeter and thermogravimetric analyser were melting points, heat capacities and thermal stability, mainly. This proposed mixture could be used as heat transfer fluid in solar linear concentrated technology extending the work range temperature between 130 and 550 °C. Results conclude that the main challenge in nitrite salts lies in the need to protect the sample above 350 °C with inert gas, to prevent oxidation of nitrite by oxygen.

Applicability of Fraser–Suzuki function in kinetic analysis of DAEM processes and lignocellulosic biomass pyrolysis processes

Abstract: In this work, a new method for fitting the conversion rate curves of the distributed activation energy model (DAEM) and lignocellulosic biomass pyrolysis process was introduced. The method was based on the curve fitting technique using the Fraser–Suzuki function. Various simulated DAEM processes were analyzed. The results showed that the conversion rate curve of one DAEM process could be described well by a Fraser–Suzuki function. According to the obtained parameters of the fitted Fraser–Suzuki functions, the influences of the DAEM parameters on the conversion rate curves of the corresponding DAEM processes can be quantitatively obtained. The experimental data of the pyrolysis of cotton stalk, oilseed rape straw, and rice straw were fitted by the Fraser–Suzuki mixture model which involves three individual Fraser–Suzuki functions. It has been found that the Fraser–Suzuki mixture model can reproduce accurately the conversion rate curves of the pyrolysis of three lignocellulosic biomass samples. The Fraser–Suzuki mixture model provides an approach to separate lignocellulosic biomass pyrolysis into three parallel reactions which link to the decomposition of hemicellulose, cellulose, and lignin, respectively.

Correlation analysis of sample thickness, heat flux, and cone calorimetry test data of polystyrene foam

Abstract: This paper deals with thermal and fire performance evaluation of expanded polystyrene (EPS) and extruded polystyrene (XPS) in a cone calorimeter with a piloted ignition. The correlation analysis of sample thickness, heat flux ( $$ \dot{q}^{\prime \prime } $$ ), and experimental results is performed. It is found that the heat flux follows a linear function of the vertical distance from the standard horizontal level to the sample. An optimization ignition model is established considering the effects of sample thickness (or radiant distance). The modified ignition time ( $$ \bar{t}_{\text{ig}} $$ ) decreases with the increase of the sample thickness. Both t ig (ignition time) and $$ \bar{t}_{\text{ig}} $$ drop as external heat flux rises. EPS’s t ig is more sensitive to the variation of external heat flux. Thermal thickness (δ P) decreases with the intensifying of heat flux, and δ P is in linear correlation with $$ \rho /\dot{q}^{\prime \prime } $$ . When sample is quite thin or the irradiance level is low (2 cm-thick PS under 35 kW m−2 and 3 cm-thick EPS under 25 kW m−2), single peak heat release rate (HRR) is present. Under other situations, there are at least two peak values. For EPS, the first peak value is higher than the last, while the reverse is true for XPS (exclusive of 5 cm-thick XPS at 35 kW m−2). Both peak and mean HRR rise linearly with the increase of external heat flux. t ig, $$ \bar{t}_{\text{ig}} $$ , critical heat flux and δ P of XPS are smaller than those of EPS, while the reverse is true for mean HRR. The ignition and heat release risk of PS drop with the decrease of external heat flux, and these hazards of XPS are higher than those of EPS.

Experimental investigation of stability and thermophysical properties of carbon nanotubes suspension in the presence of different surfactants

Abstract: Since carbon nanotubes (CNTs) are unstable in different polar solvents such as water, using surfactants can open a new gateway for solving the challenge by attaching non-covalent hydrophilic bonds. Here, the influence of different surfactants including gum arabic (GA), cetyl trimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) on stability and thermophysical properties of multi-walled carbon nanotubes (MWCNTs) in aqueous media is experimentally investigated. To reach this purpose, aqueous suspension of surfactant–MWCNT was synthesized in the ratios 0.5–1, 1–1 and 2–1. Zeta potential was used to determine stability of above-mentioned suspensions as a common method. Dynamic light scattering analysis was also employed to determine particles size distribution. The results indicated relative stability of suspensions in all ratios. It was also found that the minimum particle size was obtained in the presence of the ratio 1–1 of SDS and CTAB. Thermophysical properties of above-mentioned suspensions including viscosity, shear stress, electrical conductivity, surface tension and density were also studied at the range of 20–80 °C. The results indicated an increase in the electrical conductivity, density, viscosity, shear stress and a decrease in the surface tension (except in GA) of suspensions in all concentrations relative to pure water in constant temperature. As temperature increases, the electrical conductivity increases significantly, while the viscosity, shear stress, density and surface tension decreases more or less for all concentrations.

Quantitative analysis of accelerated carbonation products of the synthetic calcium silicate hydrate(C–S–H) by QXRD and TG/MS

Abstract: In this study, calcium silicate hydrate(C–S–H) with CaO/SiO2 ratio of 1.5 target mixture is synthesized and C1.27SH0.76 with 93.4 % purity is obtained. The dried C–S–H was compacted at 8 MPa to 20 mm × 20 mm × 20 mm for accelerated carbonation. The carbonation products and carbonation degree of synthetic calcium silicate hydrate are characterized and quantitatively evaluated by QXRD (Rietveld refinement), mass gain method, and TG/MS analysis. After accelerated carbonation at 0.2 MPa CO2 pressure for 2 h, the carbonation degree from mass gain method is 71.5 %, and from TG/MS analysis is 78.0 %. Calcite, aragonite, vaterite, and amorphous phase exist simultaneously in the accelerated carbonated samples, accounting for 33.98, 17.13, 18.74, and 30.15 %, respectively. Two mass-loss stages were observed for calcium carbonate decomposition. The first mass-loss stage (300–660 °C range) was mainly caused by the decomposition of aragonite and vaterite, and calcite formed in the accelerated carbonation process has a relatively high decomposition temperature which is mainly centered in the 660–800 °C range.

Influence of surface-modified montmorillonites on properties of silicone rubber-based ceramizable composites

Abstract: Ceramizable (ceramifiable) silicone rubber- based composites are modern elastomeric materials for fire protection application. The most important sector of economy using such materials is cable industry because there are special types of electric circuits that have to keep working in the case of fire. These kinds of composites can create ceramic phase protecting copper wire from melting under high temperature. When temperature increases, polymer matrix degrades (creating silica residue as one of the products) and mineral particles dispersed in silicone rubber matrix stick together creating stiff, durable, insu- lating and porous ceramic skin. In this paper, the influence of surface modification of montmorillonite with quaternary ammonium salts on ceramization of their silicone rubber composites is presented. Filler modification was studied, determining changes to its surface energy and thermal stability. Mechanical properties, flammability and thermal stability of composites were determined. Ceramization of the composites was discussed from the point of view of their mechanical properties and structure of ceramic phase after heat treatment, determined by compression stress tests, porosimetry and scanning electron microscopy ade- quately. Results show that type of modifier applied strongly affects properties of silicone rubber-based ceramizable composites before and after ceramization. Samples con- taining surface-modified montmorillonite produce signifi- cantly less heat during their thermal decomposition than composite filled with unmodified montmorillonite. More- over, incorporation of montmorillonite modified with ammonium salt of linear organic chain causes the creation of nano-porous structure after ceramization. On the one hand, it facilitates heat insulation, but on the other hand, high total volume of pores adversely affects mechanical endurance of the ceramic phase.

Smoke suppression properties of carbon black on flame retardant thermoplastic polyurethane based on ammonium polyphosphate

Abstract: In this paper, fumed silica has been used as smoke suppression and flame-retardant synergism in thermoplastic polyurethane (TPU) composites based on ammonium polyphosphate (APP) And, the synergistic smoke suppression property and flame-retardant effect of fumed silica in flame-retardant TPU composites are mainly intensively investigated by several methods, including smoke density test (SDT), cone calorimeter test (CCT), scanning electron microscopy (SEM), limiting oxygen index, and thermogravimetric analysis The results of SDT show that fumed silica can significantly decrease smoke production of flame-retardant TPU composites The results of CCT present that the addition of fumed silica can effectively reduce smoke production rate, total smoke release, smoke factor, heat release rate, and mass loss in the combustion process of flame-retardant TPU composites The SEM results show that fumed silica can improve the quality of char residue after cone calorimeter test All the results show that fumed silica is an effective smoke suppression agent and a good flame-retardant synergism with APP in TPU composites

Synergistic flame-retardant effects between aluminum hypophosphite and expandable graphite in silicone rubber composites

Abstract: The main aim of this work was to investigate the synergistic effect of expandable graphite (EG) and aluminum hypophosphite (AHP) on the flame retardancy of silicone rubbers (SR). The synergistic effects of EG with AHP in halogen-free flame-retardant SR composites have been studied by cone calorimeter test (CCT), thermogravimetric analysis (TG), and the thermogravimetric analysis/infrared spectrometry (TG–IR). The CCT results showed that AHP and EG can effectively reduce the flammable properties including peak heat release rate, total heat release, smoke production rate, total smoke release, and smoke factor. An improvement of thermal stability of SR/AHP/EG was also observed. The experimental observations from digital photographs give positive evidence of the synergistic effects between AHP with EG. The TG results reveal that SR/AHP/EG samples show slower thermal degradation rate and higher thermal stability at high temperature than SR sample. The TG–IR results showed that the addition of AHP and EG significantly decreased the combustible gaseous products such as hydrocarbons. These attractive features of SR/AHP/EG suggest that the method proposed herein is a good approach to prepare very effective flame retardants and corresponding super flame-retarding SR.

Differential scanning calorimetry study of peritectic steel grades

Abstract: Differential scanning calorimetric (DSC) measurements to identify high-temperature phase transitions of two non-alloyed peritectic steel grades are presented and discussed in this paper. Netzsch STA 449 F3 Jupiter thermal analyser device was used to perform DSC experiments. Measurements of temperatures of phase transformations at the heating and cooling rates of 5 and 20 °C min−1 were conducted. Measurement conditions for determining the temperature of peritectic transition in two real steels grades were described. The influence of measurement conditions on the results of DSC analysis was discussed. It was found that heating rate slightly affects the temperature of peritectic phase transition. Experimentally obtained solidus and liquidus temperatures are in good agreement with values derived by numerical calculations using FactSage software with database developed by Scientific Group Thermodata Europe. New original data (phase-transition temperatures) were obtained in this study, which may, however, be used for modelling of the solidification behaviour of peritectic steel grades.

Enhanced heat transfer for PCM melting in the frustum-shaped unit with multiple PCMs

Abstract: Thermal energy storage plays an important role in correcting the disparity between energy supply and demand as well as improving the energy efficiency. The present work numerically investigates the conjugate heat transfer between PCM and heat transfer fluid within the novel and efficient frustum-shaped thermal storage units. A two-dimensional control volume-based numerical model is developed employing the CFD software Fluent. Five units of equal volume but different dimensions are compared to illustrate impacts of geometry on melting, and the optimal geometric design is determined. Then, the multiple PCMs technology is employed to the optimal unit to further enhance the melting heat transfer. Effects of the multiple PCMs arrangement, multiple PCMs’ melting temperature distribution, and type on the melting behavior and heat transfer are investigated. Compared with the frustum-shaped and traditional shell-and-tube unit with a single PCM, predicted results indicate that the five PCMs arrangement in frustum-shaped unit can make the thermal storage time reduced by 21.5 % and 47.5 %, respectively. The great improvement may be of great significance to the optimal design of latent heat storage.

Thermal behavior study of pristine and modified halloysite nanotubes

Abstract: Pristine halloysite nanotubes (HNTs) were studied by thermogravimetry (TG) up to 800 °C. Etching of alumina from inside the tube (causing a significant increase in tube lumen) was realized by treating the material with an acidic H2SO4 solution at 50 °C. Both materials were characterized by TG-FTIR techniques and their thermal behaviors were compared with that of kaolinite. The coupling of TG with FTIR enables to detect the gases evolved during the TG experiments, thus confirming that only pristine HNTs undergo dehydration with the loss of interlayer water molecules at around 245 °C, while dehydroxylation occurs in all these materials in close temperature ranges around 500 °C. TG runs at five different heating rates (2, 5, 10, 15 and 20 °C min−1), was carried out in the same experimental conditions used for the thermal analysis study with the aim to investigate dehydration and dehydroxylation kinetics using some isoconversional methods recommended by the ICTAC kinetic committee, and thermogravimetric data under a modulated rising temperature program. Finally, the results of the kinetic analysis were discussed and explained in terms of the strengths of the hydrogen bonds broken during these processes.

Thermal degradation and flame retardancy of microencapsulated ammonium polyphosphate in rigid polyurethane foam

Abstract: Microencapsulated ammonium polyphosphate MFAPP or VMFAPP with shell of melamine–formaldehyde or poly (vinyl alcohol)–melamine–formaldehyde resin was prepared by in situ polymerization, respectively. The flame-retardant performance of rigid polyurethane foam (PU) containing MFAPP or VMFAPP was analyzed by limiting oxygen index and UL-94 test. Thermal degradation behaviors of PUAPP, PUMFAPP and PUVMFAPP were studied using TG and TG–FTIR. Above results indicated that VMFAPP and MFAPP have better water resistance and flame retardancy compared with ammonium polyphosphate (APP) in PU composites. Flame-retardant properties of PUMFAPP and PUVMFAPP composites were rarely changed after hot water treatment. Due to better compatibility and the presence of active groups on the surface of APP, microencapsulation demonstrated a positive effect on the mechanical property of PU composites.

Thermal decomposition properties and compatibility of CL-20 with binders HTPB, PBAN, GAP and polyNIMMO

Abstract: The compatibility of filler 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) with rocket propellant binders: hydroxyl-terminated polybutadiene (HTPB), butadiene-acrylonitrile-acrylic acid terpolymer (PBAN), glycidyl azide polymer (GAP) and poly(3-nitratomethyl-3-methyloxetane) (polyNIMMO), has been examined. The compatibility of the compounds has been tested in accordance with the STANAG 4147 standard and its modification consisting in the change of the heating rate. As it arises from STANAG 4147 standard criterion, CL-20 is not compatible with polyNIMMO, PBAN and GAP and possibly incompatible with HTPB. Changes of relative position of peaks between measurements performed in hermetical pans and pans with a pinhole and with different heating rate were observed. In case of polyNIMMO and HTPB, changes of measurement parameters lead to estimated compatibility change. The analysis of the thermal decomposition of CL-20 revealed that it is a two-phase process. The first phase is associated with decomposition in solid phase; the second phase is associated with decomposition of volatile intermediate products of CL-20 decomposition. Due to the complex process of decomposition of tested samples, the apparent activation energy was used for the assessment of the compatibility. The apparent activation energy of the initial phase of decomposition CL-20 and its mixtures with binders are compatible with one another within the limits of measurement error. Results of measurement of apparent activation energy do not indicate a destabilizing effect of binders on the initial phase of decomposition of CL-20.

Investigation of the structural and mineralogical changes of Tunisian phosphorite during calcinations

Abstract: The thermal behavior of Tunisian phosphorite was investigated with X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), and Raman spectroscopies and DTA-TG measurements. The XRD patterns showed that the chief mineral constituents of calcined samples are calcium oxide and fluorapatite, while those in the raw phosphorite are calcite, dolomite, fluorapatite, and carbonate-fluorapatite. Physicochemical transformations result in the oxidation of organic matter, disappearance of calcite and dolomite crystalline phases, and partial dissociation of structural carbonates. The FT-IR and Raman spectra showed modifications of some bands; a decrease in the intensity of the v2 and v3 vibrations of carbonate groups and the appearance of new bands at 520 and 926 cm−1 after calcination of phosphorites at 800 °C. These bands were assigned to isomorphous substitutions of PO4 3− in apatite by SiO4 4−. Heat treatment alters the qualitative composition of the mineral as a result; the solubility of apatite in dilute citric acid was decreased.

Thermal properties of pressure sintered alumina–graphene composites

Abstract: The work concerns the alumina-graphene ma- terials sintered by two different pressure methods. The different particle sizes of graphene were used. The prepa- ration route of the matrix-graphene mixture was discussed in the paper. The so-prepared compositions with different amount of graphene were hot-pressed and spark plasma sintered. The influence on uniaxial pressure during the sintering process on the microstructure was presented by the SEM microstructural observations and ultrasonic measurements. The material with unidirectional oriented graphene particles was prepared, and the anisotropy was even higher than 30 % for 10 mass% of graphene additive. The influence of graphene orientation as an effect of pressing process on the thermal properties was analysed. The anisotropy of thermal conductivity was 90 % for 10 mass% of graphene. The thermal diffusivity and ther- mal conductivity of composites manufactured by hot- pressing and spark plasma sintering method were com- pared. The experiment-based calculation of the specific heat versus temperature was presented in the paper. The thermal expansion coefficient was determined by dilato- metric method. The thermal stability was analysed by thermogravimetric method, and it showed that composites with up to 2 mass% of graphene can work at temperatures higher than 700 � C.

Kinetics of pyrolysis of ramie fabric wastes from thermogravimetric data

Abstract: The reutilization of the ramie-based textile waste or scraps from textile production through pyrolysis is a promising route for producing bio-fuels. In this work, the thermal behaviors and pyrolysis kinetic of used ramie fabric were investigated using thermogravimetric analysis at different heating rates of 5, 10, 20, and 40 °C min−1 under nitrogen conditions. Three model-free methods, the isoconversional Kissinger, Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) models and Coats–Redfern model-fitting method were employed to identify the kinetic triple including activation energy, pre-exponential factor, and reaction model. It was established that the Coats–Redfern model-fitting method was suspectable for determining the kinetic reaction mechanism but the most probable reaction R (R2 or R3) function can be evaluated on the basis of the activation energy value which is nearest to the value of E a obtained by the FWO and KAS methods. A kinetic compensation effect, represented by the equation lgA = −1.3515 + 0.0808E a can be observed.