Reduction Of Melting Enthalpy Research Articles

Synergistic effect of nitrocellulose coating on structural and reactivity stabilization of ammonium nitrate oxidizer

The present work aims to stabilize the room temperature allotropic transition of ammonium nitrate (AN) particles utilizing a microencapsulation technique, which involves solvent/non-solvent in which nitrocellulose (NC) has been employed as a coating agent. The SEM micrographs revealed distinct features of both pure AN and NC, contrasting with the irregular granular surface topography of the coated AN particles, demonstrating the adherence of NC on the AN surface. Structural analysis via infrared spectroscopy (IR) demonstrated a successful association of AN and NC, with slight shifts observed in IR bands indicating interfacial interactions. Powder X-ray Diffraction (PXRD) analysis further elucidated the structural changes induced by the coating process, revealing that the NC coating altered the crystallization pattern of its pure form. Thermal analysis demonstrates distinct profiles for pure and coated AN, for which the coated sample exhibits a temperature increase and an enthalpy decrease of the room temperature allotropic transition by 6 °C, and 36%, respectively. Furthermore, the presence of NC coating alters the intermolecular forces within the composite system, leading to a reduction in melting enthalpy of coated AN by ∼39% compared to pure AN. The thermal decomposition analysis shows a two-step thermolysis process for coated AN, with a significant increase in the released heat by about 78% accompanied by an increase in the activation barrier of NC and AN thermolysis, demonstrating a stabilized reactivity of the AN-NC particles. These findings highlight the synergistic effect of NC coating on AN particles, which contributed to a structural and reactive stabilization of both AN and NC, proving the potential application of NC-coated AN as a strategically advantageous oxidizer in composite solid propellant formulations.

Terminal crystalline solid solutions, solubility enhancements and T-X phase diagram of salicylic acid - 4-hydroxybenzoic acid.

The binary T-X phase diagram of salicylic acid (SA) and 4-hydroxybenzoic acid (4HBA) has been constructed from 20 °C to melting, revealing a partially miscible system with an eutectic composition of 27.3 mol% 4HBA in SA. Terminal crystalline solid solutions were obtained at the extremes of the phase diagram with solid-state miscibility limits below 0.4% at 20 °C. The limited phase boundaries could be captured experimentally by both DSC analyses at around melting temperature and solid-liquid equilibria studies at 20 °C in two solvent systems. The NRTL model was applied to regress phase boundaries and generate the final binary T-X phase diagram. The NRTL model was also used to regress solubility data, and reproduce the ternary SA/4HBA/solvent phase diagram at 20 °C and 1 atm. 4HBA was obtained as two crystal forms, viz. anhydrate and monohydrate. It is shown how the monohydrate of 4HBA is less miscible with SA in the solid state than the anhydrous form of 4HBA. As compared to pure SA and 4HBA, the crystalline solid solutions exhibited significant changes in physical properties that are relevant for organic and pharmaceutical materials in the context of impurity effects. A lattice incorporation of just 0.2 mol% 4HBA in SA caused a 10% reduction in melting enthalpy and a 66% solubility increase in 40 wt% MeOH in H2O. The reasons for this thermodynamic effect are discussed.

Copper-coated phase change nanocapsules with improved thermal properties synthesized by a green method

This study aims to develop an eco-friendly method for synthesizing phase change nanocapsules with improved thermal properties. It focuses on coating nanocapsules with highly thermal conductive Cu, using a non-toxic reducing agent. The morphology, microstructure, crystal structure, element compositions, and thermal stability of the Cu-coated nanocapsules were analyzed to ensure successful nanoencapsulation and coating. Melting-solidifying cycling experiments were conducted to assess the thermal reliability of the nanocapsules and the thermal conductivity of their suspensions was measured. The results indicated that the nanocapsules were continuously coated with Cu under appropriate reaction conditions. The optimal preparation conditions were determined, resulting in a phase change enthalpy of 77.42 kJ/kg. The SiO2 shell and Cu coating increased the thermal stability of the nanocapsules, with a mere 7.59 kJ/kg reduction in melting enthalpy after 250 melting-solidifying cycles. The thermal conductivity of the Cu-coated nanocapsule suspension increased by 26.67 % compared to that of the uncoated nanocapsules.

Experimental investigation of a thermal response behavior of d-mannitol as phase change material

The credibility of any phase change material (PCM) must be meticulously examined prior to using it as an efficient energy storage medium for any practical application. The present work focuses on experimental thermal response analysis of the d-mannitol as PCM during the charging and discharging process for medium to high-temperature applications. An 8.6% reduction in melting enthalpy during experimentation is reported while comparing characterization outcomes during the 100th and 11th cycles. However, no significant mass reduction is experienced while heating the d-mannitol at a temperature 100 °C higher than that of its melting point. It shows the thermal stability and suitability of d-mannitol as an efficient energy storage material. Infrared thermography and digital imageries complement each other to understand the thermal response behavior of the sample during experimental visualization. During charging and discharging experiments, radial inward movement of melting and solidification fronts is reported. Nevertheless, the axial movement of PCM is upward and downward in the melting and solidification processes, respectively. Analysis reveals that convection is the leading mode of heat storage during charging, while conduction dominates during discharging. Further, d-mannitol exhibits higher energy and exergy efficiency of charging (67.9% and 49.2%) compared to the energy and exergy efficiency of discharging (60.1% and 34%).

EFFECT OF Al2O3 CERAMIC FILLER ON THERMAL AND TRANSPORT PROPERTIES OF POLY(ETHYLENE OXIDE)-LITHIUM PERCHLORATE SOLID POLYMER ELECTROLYTE

Polymer electrolytes (PEs) have attracted attention in the last two decades due to their applications in electrochemical devices. At low temperatures, the conductivity of PEs is poor due to the presence of crystalline PEO regions. Low molecular weight liquid plasticizers and ceramic fillers are added to overcome the above problems. In the present work, poly(ethylene oxide) (PEO) was selected as the host polymer and lithium perchlorate (LiClO4) was used as the salt and the effects of adding the inert filler alumina (Al2O3) to PEO-LiClO4 polymer electrolytes have been investigated. Ionic conductivity of PEs was measured using complex impedance analyzer. Maximum ionic conductivity was obtained for the n = 10 sample, (PEO)nLiClO4, out of the five samples (n = 10,20,30,40,60) studied. Al2O3 was added to the n = 10 and n = 30 samples from 2.5% to 15% weight percentages. An increase in ionic conductivity was obtained when the filler percentage was increased and further increase of filler content decreased the conductivity. This is connected with the increase of the amorphous phase content in the polymer with the addition of filler. Differential Scanning Calorimetry results indicate reduction of melting enthalpy and a slight shift in the melting temperature (Tm), towards lower values with the addition of salt concentration as well as with the addition of filler percentage, indicating the increase of amorphous nature of samples. An endothermic peak is observed at 69.7 °C, which corresponding to Tm of pure PEO. The complexation of LiClO4 salt with PEO was confirmed by Fourier Transform Infrared spectroscopy studies. It can be well observed that the increase in salt concentration and the addition of Al2O3 results in decrease of crystalline nature of the polymer electrolytes.

Structural study of the Sn/Ge interface of highly milled Sn-Ge powders.

Structural changes occurring at the Sn/Ge interface of highly milled Sn-Ge powders were examined using extended x-ray-absorption fine-structure spectroscopy (EXAFS). X-ray diffraction and differential scanning calorimetry were also used as supporting techniques. Samples of varying compositions were prepared for analysis. Systematic changes in EXAFS and x-ray diffraction are observed with decreasing Sn concentration. These changes are found to be due to the fact that Sn appears in two different states, i.e., the metallic phase and an \ensuremath{\alpha}-Sn/Ge alloy phase several monolayers thick that forms at the Sn/Ge interface. When the Sn concentration is 20 vol % the alloy phase becomes the dominant phase, leaving essentially no Sn in the normal \ensuremath{\beta}-Sn phase. The results can be used to explain previously observed reductions in melting enthalpy and a low-temperature tail in the melting transition. The thermal stability of the alloy above the melting point of bulk Sn accounts for the reduction in melting enthalpy of Sn, and residual small Sn particles are the likely cause for the low-temperature tail in melting.

Raman spectroscopy of the thermal properties of reassembled high-density lipoprotein: apolipoprotein A-I complexes of dimyristoylphosphatidylcholine.

Isolated complexes of apolipoprotein A-I (apoA-I), the major apoprotein of human plasma high-density lipoproteins, and dimyristoylphosphatidylcholine (DMPC) have been prepared and studied by differential scanning calorimetry (DSC) and Raman spectroscopy. DSC studies establish that complexes having lipid to protein ratios of 200, 100, and 50 to 1 each exhibit a broad reversible thermal transition at Tc = 27 degrees C. The enthalpy of lipid melting for each of the three complexes is about 3 kcal/mol of DMPC. Raman spectroscopy indicates that the physical state of lipid molecules in the complexes is different from that in DMPC multilamellar liposomes. Analysis of the C-H stretching region (2800-3000 cm-1) of the complexes and of the pure components in water suggests that below 24 degrees C (Tc for DMPC) there is considerably less lateral order among lipid acyl chains in the complexes than in DMPC liposomes. Above 24 degrees C, these types of interactions appear to contribute equally or slightly less to the complex structure than in pure DMPC. The temperature dependence of peaks in the C-C stretching region (1000-1180 cm-1) reveals a continuous increase in the number of lipid acyl chain C-C gauche isomers over a broad range with increasing temperature. Compared to liposomes, DMPC in the complexes has more acyl chain trans isomers at temperatures above 24 degrees C; at temperatures above ca. 30 degrees C, trans isomer content is about the same for complexes and liposomes. A large change was observed in a protein vibrational band at 1340 cm-1 for pure vs. complexed apoA-I, indicating that protein hydrocarbon side chains are immobilized by lipid binding. The Raman data indicate that the reduction in melting enthalpy for complexes DMPC (approximately 3 kcal/mol) compared to that for free DMPC (approximately 6 kcal/mol) is due to reduced van der Waals interactions in the low-temperature lipid phase.